Sadly the original divorce decree allowed for the split on the dependency exemptions so I understand that even though he gave up custody he doesn't also want to give up the exemption that was already decreed. Especially if he pays the child support timely. Sometimes blowing into the wind is not worth the effort mentally or physically.
In the end the mother needs to be the bigger person and not nay say the dad in front of the kids. Trust me they know what is happening even if you don't think they do and will think more of mom if she is kind hearted and forgiving. If she does this she wins in the end where is counts.
Last thought ... make sure he is paying his fair share of child support ... that is worth the cost of an attorney to get the amount adjusted if you believe he is under paying.
