And Moshe called Hoshea the son of Nun, Yehoshua. (13:16) Rashi says that Moshe prayed for him may Hashem save you from the plot of the spies.and he arrived in Chevron. Rashi says Calev alone went there to pray by the graves of the Patriarchs so that he should not be persuaded by his companions to be part of their plan. Were not Yehoshua and Calev the strongest in faith among the spies? Were they not among the leaders of Benai Yisroel? Why did they need extra Teffilos to protect them from the plot of the Meraglim? We see from here how dangerous outside influences can be. No matter how strong a person is in his observance of Torah and Mitzvos, if he associates with the wrong crowd, hangs out with the wrong people, there is no telling what can happen. A person must be careful who is friend are and who he associates with. So too, a parent must monitor who his children's friends are. (Rabbi Yechezkel Levinstein, Yad Yechezkel)
