He's needed help since childhood, it sounds like. You have been a good friend to him for over a decade and a half and you have been his sole anchor in life.
But yeah, he's got massive issues, and this is kind of what happens when they go untreated (note: I am not a doctor). He seems to fancy himself as some sort of white knight, but because he's got lousy self-esteem, he picks the bottom of the barrel for his reclamation projects.
Frankly, I'm surprised no one's rolled him or otherwise stole from him by now.
He avoided talking to you for the exact reason you determined - he knows there are problems and he knows you would not approve. So he also knows what he was getting himself into, or at least he should have had a clue. At some point, a person needs to start learning from their behavioral patterns. It seems to me that he is getting lessons just fine, but he is not heeding them at all. He seems to feel the next one will be the one who doesn't screw him over. Yet he continues to seek out relationships with people who will do just that (and it's generally in their own best, selfish interests to do so). He seems to feel he deserves it.
It's also unfair if he has been treating you as a therapist all these years. It's one thing for you to be his kind confidante. But if he is using you as a therapy substitute (and I think he is), then that's preventing him from actually getting the help he needs.
There's a few ways you can go with this, listed in no particular order:
- Do what you have done for years, which is let him go and do his self-destructive nonsense and be friends again when he returns.
- If you do the above, consider finding ways to either get him into therapy or get him to meet people who aren't so self-destructive. It might help him to see that there are options out there for people who might care for him and, at the same time, not want to screw him over.
- Make a conditional break with him where you tell him you're done unless he can get therapy.
- Make a clean break with him. Tell him you're completely, 100% done with him.
You can potentially do the first three all at the same time. And then if it doesn't work, go to #4, which is the nuclear option.
I don't envy your position. It's hard to see the people we care about repeat self-destructive patterns, over and over. But they may be using us as props to hold up their difficult lives. If we remove those props, those people might see that things are not fine, they are not all right. Then getting help becomes the only option to them.
I liken him to an addict. He certainly seems to be behaving like one. Usually the best thing you can do with an addict is isolate them except for the one path, the one to rehab and recovery. Not all of them make the right decision, unfortunately.
I hope this guy sees the light before he wastes any more of his life on people who are 100% not worth it.