Duct Tape his Face!
Rehab but they have to want to quit, trying to force someone into it may result in a relapse not always but usually. It usually takes them to hit rock bottom to really try to get help but if they have enough support behind them they can be successful in getting clean. Be supportive,caring and understanding in helping them getting help to get clean. Good luck!
You can't stop someone from this. They have to make that decision for themselves. Just be a good friend to them and try to show them other ways to deal with the pain of their life. More positive ways of dealing.
Give them some powder coke and a spike.