It's rare for a person to reach out for support and simultaneously admit to all the things they're doing that are contributing to the problem. This isn't just because doing so demands a great deal of humility and emes; it's also because it's very hard for anyone to see their own marriage objectively.
I have no reason to suspect that anything other than what you describe is happening here, but it's clear that there are potential distortions or misrepresentations in the narrative, especially as you are hearing about things second-hand. While I'm certainly not accusing anyone of deliberately twisting the truth, there's only so much that a person can see from their own vantage-point.
Therefore, I want to stress that I'll be offering advice for someone who fits the exact description you provide - namely, a wife who is doing her best in a difficult situation with a husband who has multiple issues, and who is seemingly not directly contributing to the challenge. If that turns out to be an incomplete or inaccurate assessment, the advice I give will probably be inadequate or even inappropriate. (Similarly, somebody trying to apply my advice to their situation may not be helped if their situation is at variance with what I described.) Perhaps your friend has her own issues and projects them on to her husband. Perhaps both her and her husband have contributing issues. It could also be that she's exaggerating (perhaps without realizing it), or that she interprets simple conversations as dramatic confrontations, as this is the way she experiences them.
In general, it takes great talent and intuition to assess a situation properly. It's easy to jump to conclusions based on a narrative; in fact, professionals often disagree among themselves even after hearing from both spouses. How much more so do friends and relatives have to be so very careful before making their minds up. For example, you write, "From hearing about the situation I understand that the husband has some mental health issues, to do with anxiety and emotional regulation." I've seen so many incorrect diagnoses of mental health issues made by those who only heard about the situation second-hand. Such mistakes can cause immense damage and heartache.
Furthermore, even if the diagnosis is fairly accurate, being told that one's spouse has a "mental health issue" is not usually helpful. Most people, when they hear, "He has x diagnosis," become resentful and start to regard and treat that person with less respect, which obviously has less-than-wonderful repercussions. Worse still, most people who hear that a person has an issue interpret it as a life sentence. After all, "everyone knows" that there's no cure for a "mental illness" or "personality disorder."
In too many cases that I have dealt with, the "mental health issue" turned out to be more of a conflict of personalities where the "healthy" spouse decided that the other was seriously imbalanced. This is more likely to occur when one spouse has a more sensitive nature than the other. Yes, it can be hard to live with someone who is much more emotional and sensitive than oneself, and a person in such a situation may need advice and help in dealing with the unique challenges it involves. But there's no illness that needs "managing." Furthermore, when the issue is properly understood, all can be excellent. Yes, excellent.
Back to your role: You can provide the necessary help and it's wonderful that you are trying to. Everyone needs a good friend who can listen and offer helpful advice. But, unless you're a professional in this regard, giving useful advice when there's only so much information you're privy to is very hard. There's a big risk of unwittingly doing a lot of damage even when you mean so well.
Even a comment such as, "You're doing amazingly in such a tough situation" can cause harm as it basically confirms in your friend's eyes that she's the "good" spouse and her husband is the "bad" or problematic one. It can also subtly transmit the message that if at any point she finds it hard to do the right thing, that's understandable and forgivable.
In general, seeing things in black-and-white terms of right and wrong, healthy versus unwell, is rarely productive and often damaging.
Let's take a deeper look at this situation in order to answer another part of your question. Let's say your friend is right and her husband is "the problem," due to the fact that he "can't control himself." So why do you expect him to be part of the solution? If he isn't in control of himself, how will it help to force him to go to therapy? I know people tend to hope and believe that a therapist will perform magic, and I wish I could say that overnight, dramatic change can happen as a result of therapy... but sadly, it just isn't so. Perhaps, instead, if her husband is truly overwhelmed by anxiety, his wife would do better to accept his limitations and get the help she needs to deal with reality?
Many people are delighted when their spouse, or child, receives a diagnosis. For some reason, they feel that it absolves them of responsibility: "He has issues, and so l..." The truth is exactly the opposite. If your spouse has issues, if he's the problem, then you have to be the solution. There's no one else.
This isn't a message of doom. Certainly there are people who consider themselves victims of a difficult spouse or child. But they could equally choose to see themselves as empowered, as privileged to be in a position to help, as grateful for the koichos they have been given to support the vulnerable, emotionally fragile people around them. It really is a choice of perspective, and one with significant implications.
And this perspective shift is relevant for her husband too. If she sees him as "the problem," he will feel it. If, however, she sees him as a person with vulnerabilities as well as strengths which is what he is she will be able to continue to respect him and he will feel that respect and respond positively to it.
Most people have never considered this idea - that their spouse is constantly reacting to the way they are perceived. When we think about it, it's obvious of course someone who feels respected will act in a more respectable way. (If they don't, that's a sign that they are genuinely troubled.) You write that his explosions aren't "because he wants to be nasty" and that he is "a nice person who loves his wife," but does he have any idea that his wife sees him as nice and loving? Imagine for a moment, or you could even ask your friend: "If someone asked your husband what he thinks you think of him, what would he answer?"
This is why it's important for your friend to identify the areas in life where her husband demonstrates his strengths, rather than focusing only on his weaknesses. She's told you all about the situations in which he feels overwhelmed or anxious, but what about the situations where he shows calm responsibility or genuine caring and affection? Could it be that when he feels respected and admired, his good qualities shine - and that when he feels diminished or even despised, all his negative traits find expression?
Your friend isn't responsible for ensuring that her husband only encounters people and situations that bring out the best in him, but if there are simple things she can do to help him function better, why not?
When it comes to inevitable disagreements in the home, there are certainly ways to avoid the kind of scenes you describe. It's true that she doesn't "have" to ask about every little household expense, but she might be surprised to find that if she does ask about the things she knows annoy him, it won't be long before he willingly agrees and even tells her that she doesn't have to ask his permission anymore. So often, once a sensitive person senses that their feelings and preferences are being respected, they ease up a lot and don't feel so pressured to assert themselves in unhealthy ways.
For example, if she feels that an extra nosh or salad on Shabbos is important to her, she can share those feelings and ask if he's okay with spending the few extra dollars, and then tell him how much she appreciates it. If that's what it takes, why not...?
On the flip-side, while it could be true that he finds appointments highly stressful and might simply not go without his wife at his side, this doesn't make it her responsibility. His appointments are in the "his" sphere of his marriage, not "hers" or "ours." For her to feel obligated to cover for "his" area is a blurring of boundaries. When boundaries aren't respected, problems almost always follow.
If she really doesn't mind going, she may go. But it should be clear that she is doing so out of choice and that the responsibility is his. When she does mention this, of course, she should do so respectfully and calmly. And if she doesn't want to go, she should communicate her decision without seeming uncaring.
Your friend should also accept, however, that just as her husband's appointments are his business, so too is his decision regarding whether to go to therapy. If he doesn't want to go, no one has the right to force him no matter how problematic they think he is. In any case, it's very unlikely that someone forced into therapy will find the experience productive.
But if your friend wants to go for professional help herself, she can insist that she needs it. She can and should calmly tell her husband that it is important to her and that she hopes he will understand.
And if he responds to this new approach with anger, she doesn't have to excuse his behavior or silently absorb it. Very often, people with poor emotional control allow themselves to display bad middos partly because those around them let them get away with it. This is a topic that deserves an essay of its own; for now, let me just stress that your friend must learn how to stop allowing him to behave in unacceptable ways, without becoming aggressive or confrontational.
Meanwhile, try to take a step back and take stock of the direction in which things are moving. If you notice your friend gradually finding it easier to accept her husband as he is, with more confidence that she can deal with the situation, that's great. If, however, she starts becoming more resentful or demanding, or immersed in self-pity, it would be wise to persuade her to find someone who will encourage her to see both her and her husband's strengths and build on them. Once she starts to focus on his good points, it will become easier for her to show him respect.
That said, showing one's spouse respect is a basic responsibility in any marriage, regardless of the circumstances. Nothing justifies showing disrespect, and even if someone may not have chosen to be in a high-maintenance marriage, if that is what Hashem gave them, they can be sure He also gave them the strength and wisdom to deal with it. Therefore, do what you can to help your friend not to see herself as a victim and to feel good about becoming part of the solution. If she invests wisely and happily, she will see b'ezras Hashem tremendous growth, together with her husband.
