If one of the children is over 18, does that mean that the other parent no longer has any legal obligation to pay 50% of the out of pocket expenses?

If a child of divorced parents is 19, college student, and still covered by one of the parents health Insurance, is there a legal obligation for the other parent to cover the 50% of out of pocket expenses that that child's healthcare needs incurs? Is that something that would have to be brought before the judicial system? I insured our children, am a teacher and make $41,000. My ex does not carry insurance coverage for them, and makes $115k, but is to pay 50% of any out of pocket expenses. With my insurance, I can cover my dependents until they are 26. My ex’s stance is, she's over 18, not his problem. Therefore, I'm holding $27,000 of out of pocket expenses that my insurance has not covered for a recent medical issue regarding our daughter. Please help with advise.