Have you or your friend tried talking to the violent members of his system to find out why they act that way? I think that would be very important - they probably need just as much help as your friend does, and once you know why they're upset your friend should have a much easier time dealing with them. Although if they refuse to talk and just insist on being violent without giving a reason (or any indication that they plan to stop being violent any time soon) - that could be a big problem.