You should first talk to the guy. Ask him if he's feeling all right or is okay with the way his friends treat him. Take note of his answer and see if it correlates to what you see. Keep an eye on him.
If things are obviously not good, tell the guy that you aren't liking what you see and that he should stop associating with his friends. If nothing improves, talk to someone with authority who may be able to help you.
If worse comes to worse, stand up for him yourself. If you feel that what's going on isn't right, then you have the right to speak out against it. It's never right for other people to treat other people poorly.