child support arrearage

Does anyone know what the law is in Colorado about waiving arrearage balance? My husbands ex waited until the child was 3 to file child support, and he was not given any credit for the monies that he was giving her, so he started with a huge amount of back pay. We paid it down substantially and asked if she would consider dropping the rest because she wants someone else to adopt his child. Also note that the child is 7 and has no clue who his dad is because Mom thinks he is too young to understand. She tells me that she can not drop the arrearage until my husband has relinquished his rights, and that does not sound right to me. We live in a different state, but several women have signed the arrearage away and maintained the current support order, which is all we are asking.