To be honest, id want to know where my children, when not with me ,were living. Surely that's a basic parental right?
If I was your daughter, i'd try and talk to her ex, try to be as amicable as possible with him. I know it's probably easier said than done, but surely she must sympathise a bit how hard it must be for him? It would also make life that little bit easier and better for their son if he can grow up knowing his parents were being 'adult' about their situation.