I would never date a guy like that. Fool me once shame on you, fool me twice shame on me. I'd assume either the guy was a total train wreck or had the absolute worst judgment ever and neither are qualities I find appealing because I don't want to be a part of someone else''s learning curve. Don't get me wrong, I get that some people have stuff going on and I can be sympathetic about it, but the OP asked the question "If you were dating"... not if you were friends with ect
If you really want to make her take a look at him through critical eyes I think you need to be as sweet as pie when you meet her, which will throw suspicion on him when he bad mouths you to her. After all, one of you has to be messed up, it it isn't you who else does that leave?
Nothing you do or say to any woman would every pry her off some guy she is convinced is her Prince Charming. Most woman believe in fairy tales and that it all went wrong before just because it wasn't 'meant to be'. I say save your breath and count your blessings that he's gone.
Still, on the less cynical side: One of my cousins, a beautiful normally smart woman, married a guy who had been divorced 3 times, making her #4. He was pretty well off but had 3 families of kids he was supporting. When they met in the late 80's she was recently divorced working as a school bus driver with 2 young kids. They have stayed married probably because, go figure, my cousin had a hidden talent as a saleswoman and started making lots of her own $$$ so her husband could goof off all day and play golf. I thought she was crazy when she married him but they're still together and she seems happy, and he helped her raise her kids to be successful, one's an Dr and the other is a clothes designer... who can say what will work or why. Sometimes things play out in the strangest ways.