Signing a document under false pretense
My fiancé is having a custody issue with her ex husband. Currently they share custody 50/50. They found out that he owed her about $12,000 in back child support so he agreed to go to a mediator and give her the majority of the custody time if she did not go after back support. He would not go to the mediator until she signed the paper, so she signed it under distress. Now he does not want to give up his custody time and in fact wants to take time away from her. Is there anything she can do to get the paper she signed voided since she did not get what she was promised? All she wants his full custody of her son, she doesn't even care about the money. Thank you for any help I can get.