Poor guy. I've seen this in my own family. I doubt they can work it out and it will only get worse with age. She will definitely not take kindly to him wanting to take any of the kids either.

In theory, one should start by looking for physical causes - medication side-effects, disorders of the endocrine system, etc. Then quietly ask whether or not she's n=1, in her known blood relatives, for such behavior.

Also, while actually talking with your kids about their mother's issues is currently a no-go zone - kids are quite adept at reading emotions. One can both acknowledge that you're seeing the same Big Problem that they do, and that divorce is also generally horrible - without saying a word.