Custodial Parent Does Not Give Child Support Kids

My situation is that my fiance pays child support to his ex for two children but we are sure she is going to take him back to court for more child support. The problem we have is she lives with her parents and hasn't held a job for more then a year and does not help in the support of her children. She only supports them on the father's child support. Isn't she obligated to get a job and help support them too. Why does he have to support them by himself when she just sits at home and spends the child support on herself. The kids still come to us and ask us for money because she doesn't never have any because she doesn't work to help support them. So my question is does he still have to pay the full percentage when she doesn't work, doesn't have a car and her parents don't have a car to transport these kids when they need to go somewhere. She has no physical reason she can't work, she just doesn't want to. I thought they would both be responsible for the welfare of these 2 children.