Well, she's history, he needs to think about what's best for the daughter.
I went through a divorce with a young daughter and we sat down and agreed that we both loved her and she loved both of us so minimize any negative impact as much as possible. Neither ever bad mouths the other. I respect her as the only mother my daughter will ever have, even if she is a dipwad most of the time.
I suggest he do the same. There is NOTHING that woman does that releases him from doing what's right.