What is the ruling concerning a friend to whom a fatwa prohibiting sitting in places of sin was sent, and then he went back on his agreement and sat there, and what is required regarding friends who have done that?

You have done well in your eagerness to prevent yourself and your colleagues from sitting in a place where forbidden acts are committed. As for your friend who erred in his behavior, continue to advise him wisely. As for your other friends, strive to guide them as much as you can, but if you fear that you will be negatively influenced by them, then distance yourself from them.