I have a friend who is divorced in PA. At the time of the divorce, they agreed to carry health insurance on both children (written in the divorce agreement.) My friend is the primary, and his ex is the secondary. Their daughter was just hospitalized, and my friend found out his ex no longer carries insurance on the children. He was socked with a three thousand dollar bill from the hospital that his insurance did not pay. Normally, his ex's secondary insurance would have pick that up, however, since she no longer carries insurance on the children, who is responsible for paying that bill? Would it be her, or would they split it 50/50?
It is very hard to tell with what you have provided. Is there a court's order regarding custody? It appears that they both my be equally liable to pay the costs. Your friend should get an attorney and get back to court to enforce the divorce agreement along with reimbursement. A bigger issue is what is your friend doing to get the children insurance coverage? If he is paying child support, and pays for insurance he may be able to seek a deduction of his support as well.