Can an ex have a Bill Collector bill you for Medical Bill on a child when they registered them...?

My husband's ex is having Bill Collectors send my husband the bills for their children. She register and takes them to the hospital for services and yet is able to have them bill my husband after it's been in collections and she refuses to pay. In the divorce decree my husband is ordered to pay which he has paid more than half because we have custody of his son and pay all his medical bills and send her statements and she never pays us half so when she has a bill we know how much were up on her and some bills we do not need to pay. Well she has obviously found a loop hole and started having the Bill Collectors just send us her bills. She hs them change the bills into my husbands name and we do not owe her half on that bill as she has not paid us half on any of their son's bills. We keep a ledger showing all medical bills between all kids and keep track of our payments. Can a Bill Collector act as a Divorce Mediator, they do not know what she has and has not paid on our bills for their child. If ayone has had this same situation we need some advise. We cannot pay all the medical bills on our own and pay for Medical, Dental and Vision insurance. We agree to pay our half but not her's too. So frustrated, I feel like she will always try and manipulate us this way and we cannot afford this.