It's hard to say what you should do without knowing the other guy. Do you think you could talk to him and to get him think about what will follow if he slips again?
My cousin is pretty much in the same state, but with alcohol. He has a 10yo son who means everything to him, yet he cant stay off the booze. He spent his new year's eve and christmas in rehab for the umptheenth time, he has his good times, usually during the summer when he can be on our cottage away from booze, but come winter and it's rehab time again.
Needless to say we are all extremely worried about him, hell his the best guy I know, always there for me and my brother if we need someone to talk to or just have hard time otherwise.