I have a question about child support in texas...please read on..?

I have a friend that owed back child support for 2 years. Him and his ex came up with a deal that she would drop the money owed (for those 2 years) and they went to the court and signed an agreement that he was free and clear for any back money owed. Now 6 months later she is changing her tune and is threatening to go back and reinstate the 2 years of back child support. (even though he has been paying what he owes the past 6 months & then some) Can she legally reinstate what she initally signed? and he would be forced to pay for the 2 years? if this question is confusing please let me know! thanks for your help!