But here's the thing: I'm totally down with "not throwing away history", but that's not to say that you need to cultivate the abuse. There are no rules here, and there are no absolutes. There's no reason you can't create a little distance for your own (maybe temporary, maybe more permanent) well being.
I'm a big fan of the concept of the "emotional bank account". If you spend spend spend, without replenishing in any way, it's only a finite resource. I find that I spend less and less time with those that continue to withdraw without any replenishment. It sounds like your friend HAS replenished throughout the years, and that's good. But maybe while he's in withdrawal mode, you just keep a (maybe temporary, maybe permanent) hold on the pursestrings.
(And I'm just being honest here: I'm open-minded when it comes to substances. I love my liquor, I judge no one on their ganja intake, or muchrooms, etc. I even had a girlfriend who's brother was big into horse, and it was what it was. I have ZERO good experiences with cocaine use among friends. It's the devil's dandruff, in my opinion, and also in my opinion, it changes people, and not for the better. I don't mean "just brings out other aspects", like liquor does, but actually changes people. I'd consider that as well as you plot your next move.)