rather than love, than money, than faith, than fame, than fairness... give me truth.
I think to fix it:
File for visitation or set up a schedule with her
Stick to that schedule and if she needs him he sees his child when he is not occupied but his visitation schedule includes tons of time where that is reasonable
If I read your tag line correctly when kids are involved we go to the ends of the earth. He may continue to be pulled by her but you have your own children and focus on what works best for you and your kids. It is disrupting but I would try helping and if it doesn't get better then decide whether to move on or just not plan on as many activities with him.