Someone doesn't change that extremely without some cause. It might be a chemical imbalance, it might be ... oh, all kinds fo physiological things. But it also might be a stupid whim that turned out to be a mistake and not knowing how to fix it.

I understadn your taking sides, but you REALLY need to figure out how to get your Dad to some OTHER support system. Maybe there's a support group at his church for widowed and/or divorced people. Maybe there's ... well, he needs his KIDS to be his KIDS, and to replace the support system that he had for so long (sounds like his SOLE emotional support was the stbx). Please don't get me wrong, kids are a good support, but it's not a good thing to allow the roles to be reversed over this type of thing. When he's so old that he's physically disabled and unable to feed himself, the roles will be reversed, but he needs to feel like a strong man who handles things for the family & kids, so you need to point him in the right direction and then back off.

Your mother has made some serious mistakes and I'm not diminishing that, just that because of the special relationship you have with your parents, you should NOT be the one trying to fix this. As I said, point him in the right direction to get help and advice, and then let him make his own decisions.

KGrow is right, if his marriage is like most, he'll be lucky to get out with the settlement you've described, but he won't know that till he talks to a lawyer in HIS jurisdiction (or your mother's, maybe, if she files there), to find out what the numbers look like.

Again, good luck.