To answer your original question, I think that the odds of THEIR relationship surviving or spiraling further out of control are impossible to guess considering they both seem to have enormous issues with dependency. (& probably co-dependent enabling)
I also get the sense (although not from anything you've said) that you think HE is her problem & that if she weren't with him, her life would become more manageable. She is an addict. Simply ending her current relationship isn't going to set her life on the straight & narrow & get her clean & sober. Only SHE can make that decision.
I'm wondering though, if you are hyper-focusing on HER relationship & HER issues as a way to avoid dealing with the fallout from your OWN divorce? I think it's a perfectly human reaction, but wish you could see that you'd be shooting yourself in the foot if you don't take this time after your divorce for yourself without getting sucked in to ANYONE else's drama.