Custodial Parent not paying the medical co-pay

My Husband has been divorced for 3.5 years and during that time his 2 boys were not on tricare prime so his ex wife were getting all these small bills at her address and not telling my husband. We only found out when we check his credit report (18 hits of medical).
We knew she was the one responsable for any other medical bills after the divorce.
SO after consutling the divorce lawyer and also another women who is my friend but has her own practice. We were informed that if a divorce decree only has that my husband is required to pay his child support and nothing about medical it is the custodial parent who pays any bills that come up.
Because the boys are on tri care all of it went on his credit so we paid them over 600$$.
(yes they are now on tricare prime)
She lives in upstate NY and we are down in NM, I will be u there and want to try to get this taken care of.
Family court told me to file a modification of support but filling that out, it dosen't apply to this situation.
Has anyone had this issue and if so what did you do to have the ex pay back the bills we had to pay. Oh and my husband has never missed a child support.