Pay it back. Sorry to be so blunt, but if you have to go back to court for what your ex owes you this could be twisted around on you. It would be wonderful for you to be able to say thank you with grace and accept the gift, but you can also graciously decline and say you feel better about paying it off. This woman has done so many underhanded things in the past........especially for her son and I just don't trust her. They tell us to listen to our gut feelings.........you're my friend, I know what's happened, and my gut says this isn't something done out of the kindness of her heart.
I know there is some good in all people, but given the situation............not this time.