Been there and done just about everything in a HUGE way -still managed to come out the other side and make a success out of myself. Only way to escape from all of that is to blank it completely, either he goes or you do - otherwise you'll end up miserable or wasted. The alternative of course is to tell your mate that you don't want any smoke in the house incase you fall off the wagon. It's very hard to get yourself straight but its very easy to lapse back into getting smashed all the time.
The band I was in during the early 90's lived the GNR lifestyle for real, I managed to get out and sort myself out before it was too late - found out a while ago that the other guitarist (my best mate at the time) died from an OD two years ago (guy got badly hooked on smack and coke after he decided £150 worth of hash per week and a bottle of vodka+ per day wasn't enough). The other dudes from the band are all misfits now too, glad I got sorted because now I'm married with kids and run a very successful business. Seriously take up weights, the endorphins make you feel a lot better and when you get fit the last thing you'll want is to smoke your brains out :) Stay strong!