I'm not sure I agree, actually. Although normally I would be the first person to say cut him off - in this situation, he's clearly very very clinically depressed. And his behavior towards you is not really the norm - usually they're crappy to you when they're in the infatuation with the OW.
You need to have healthy boundaries, of course - but he's a drowning man reaching out to you because you're his life raft. I'm not sure I'd cut him off in this state. He's been through an extraordinary experience that none of us can fully understand and he has a freakin' brain injury.
Now - it would be fair for you to consider whether you want to make a life with a man who is so damaged. Sadly, the man you married may never return. And no one would fault you if you decided you'd had enough. But even so, it's ok to be his friend. This all so clearly has nothing to do with you.
Btw, it IS true that sometimes what snaps them back is the realization that you are moving on and that some other man might snatch you up. But I would tread very carefully here - he sounds like a suicide risk in his current state.
