How can one deal with a colleague who engages in foul and obscene language, yet is a devout and observant person at home, but changes at school due to bad company?

Advising a friend involves several things: speaking with the student counselor, the religious education teacher, or one of the friend's wise relatives to help you in advising him. If the friend does not respond to the advice, continue to associate with him as long as it does not affect you. But if you feel affected, reduce your interaction with him. Increase your supplications for him, and continue to advise him with gentleness and kindness. Do not overwhelm him so that he does not tire, and do not be so brief that he forgets the admonition. If you are able to advise his companions and influence them, that would be better.