If it is in the agreement that he pays medical costs, he pays. If the agreement says medical costs are embedded with the child support payment of $2,000. then she pays. The parents need to talk about unexpected costs and how to handle them so that the child benefits. Re-read your agreement.
My ex and I made sure we both carried our son on our health insurance policies to prevent this. Is this money not covered by health insurance? For example, 20% left over what insurance pays ? Or was there no health insurance at all? Because if there wasnt, the hospitals jack the rate up 20-50 fold of what a health insurance company would have paid them for reimbursements. So it will be a LOT of money. This now means both the parents credit rating ruined, especially if she out you down as the responsible party to pay. Then you'll have to deal with the hospital administrative office on this to protect yourself. So find out from an attorney what your rights are and make sure the attorney tells the hospital if its found you dont have to pay.. For the sake of your child, try to work out something with the ex.