Collection agency question and big problem

I have a few medical bills that were turned to a collection agency on my daughter. By the divorce decree her father is responsible to pay half of all medical bills on her.
I have paid my half and yet they are still harrassing me for payment of these bills.

This bill has a small balance of $200, but since I have paid my part I am reluctant to pay more just to cover his bottom. They say they will take other measures to get this money. But I am and have done my part why haven't they tried to get the money from him.
Any one with the answer as to what I do next please tell me.
This collection agency also has preceeded to call me at work even though they have been told not to.
And she used bad terms of collecting with me, by telling me that I was not handling ly life right by not paying my bills on my daughter. And ruining her name. By having it in the collection database.
SOMEONE PLEASE HELP ME!!

The collection agency doesn't care who is supposed to pay the bill. Your divorce decree means nothing to them. They will get their money from the most likely source, and that's you. If you are able to pay the bill, pay it. Then take your ex to small claims court and sue his butt for the money he owes you. After you get a judgment against him, you can do fun things like garnish his wages and his bank accounts and file liens on his property. You might also try dragging him back to divorce court on contempt charges for not living up to the decree and paying his half of the medical bills.

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I'm not a lawyer, just an educated consumer.

Whatever you do about the bill, do not let the collection agency harrass you. Send them a letter ordering them to cease and desist from any further phone calls to you at home or at work. Mention the Fair Debt Collections Practice Act and at the bottom of the letter, put a cc to the State Attorney General, Office of Consumer Affairs (you don't actually have to send the Attorney General a copy). Send it certified, return receipt requested.

If they call again, tell them you are recording the conversation and again, order them to cease and desist. good luck to you.