Pretty much. The "minding my own business" game is horse shit.
Are you fucking the girl as well or something?
This is seriously the most fucked up 12oz tale I've ever read.
Don't get some other girl to fuck him, that is a bad idea and the last thing he needs at this time. Clearly regardless of the circumstances he will become infatuated, thus creating a whole new spectrum of pain and an even worse situation.
You need to put that women in check, if physically then so be it and this is the only time I think I'd ever say that.
If your friend is cognative enough to talk to you about what is going on, I'm sure he can get professional help which is the real solution. If he really has a mental deficiency there are TONS of FREE care programs to look in to I'd imagine; However money doesn't sound like an issue. Also, you ARE a witness to this, and could see to it that appropriate legal action be taken.
Finally, don't fucking smoke weed with him! Tell him to cut that shit out, the last thing anyone like that needs.
Also, I'd take anything he ever bought for her that is still in her posession, shitty music CD's included, and put it on Ebay and give him the loot.
I was almost in tears reading that, nobody deserves this.
So quit enabling it.