Violated payment agreement

My sisters husband agreed that he would pay $5600 for marital debt during their divorce. He agreed to pay her $400 month until it was paid off. He has never made one payment. what can she do? Also, since I am going thru divorce also, I have about $20K in marital debt in my name, I know he wont agree to anything and so we are going to court. If a judge orders he pay me the debt, and he doesn’t can I do anything? Just preparing.

She can sue for breach of contract.
If your ex does not abide by a court order, you can file a motion to show cause and the court will enforce the order.