personally speaking i dont find anything wrong with her meeting her ex once in a while. I think if the girl and ex-husband have a good relationship, it will be advantageous for their children. Also if the ex husbands new wife seems to be fine with it why are people complaining.
Islamically speaking her ex is not a maharam, but as long as she observes hajab i think it is okay as they would need to discuss things about the children.