Ugh. I hate hearing about stuff like this. I hope he gets out and finds a better situation.
I had a pretty big falling out with a cousin who was in a situation like this. When she finally left we became close again but I couldn't keep my mouth shut and she couldn't hear what I was saying. It is hard.
There is nothing you can do to make them leave. Trust me...I was there, I wouldn't listen. Finally I had enough and got out. It had to be MY decision... but please please please be a friend and be there if they need you.