Could she get the back child support?

My fiancee and his ex wife got divorced back in 2003. The state wanted 235 a week but the mother said that was too much and wrote up a letter stating that she only wanted 125 a week. She signed the paper as well as my fiancee. They never went through the court. So now after 6 years of not being together she wants to take him to court and sue him for over 43,000$ in back child support? He has ever check copied that he has ever given to her. I just find it funny that after 6 years shes just now saying something? Her and her new husband are filing bankrupt and might lose there house? Could this work in for favor?