The only positive about OWUmpteen in my girls lives is that he seems to still be working really hard to pretend to be a decent father around her. Better the Whore you know, IYKWIM.
I think you are borrowing trouble. I can't say I won't be doing it myself when the time comes and I too feel sick when I think of the parade of 'luuuurves' he is going to subject my children to. Not to mention modelling love addict behaviour just as his mum did to him. He will accelerate the self-destruction - not that he's not doing it now its just that it will be more obvious. Everything they've done to stave off their uncomfortable feelings will come crashing down around them and impact our children by proxy.
When the time comes I hope I can muster a bit of hope for the best, expect the worst.
Sending you as much mojo as I can muster that your prediction is incorrect. Chances are it is not - control/influence what you can, surrender to the rest.
The silver lining here is your son is soon able to choose where he spends his time. If that doesn't change her behaviour then he'll have to make a choice. Not something any of us wanted for our most precious but unfortunately these are lessons they will learn no matter what we do. It is a big bowl of suck.
But seriously, how nice would it be to be parenting with a functioning adult? Someone who can cope with life?