You may not be able to stop him if he's determined to go ahead with it and is disregarding (what look to you and me like) obvious danger signals.

I'd say your best chance -- of living with the outcome OR, by some miracle, still intervening -- would be to see if you can figure out more specifically (1.) what he might see in her and (2.) why it might be so important to him that he'd overlook all the rest of what you've described.

Whatever it is, you might not be the one for whom it's easiest to spot, since you're so close to the situation and you've apparently had other issues with your dad besides his choice of women.

(If you did get him to change his mind about her... well, I wouldn't hold my breath, but I certainly would watch my back!)