this exact thing happened to someone I'm close with. After a long battle of trying to get custody of his kids from a

You cannot have a formal court ordered agreement saying that you agree to not go after the other parent for child support if they relinquish their rights. If you have an informal agreement the other parent can go after them at any time down the road and legally demand child support.

that’s a myth, you do not get out of child support in most states by giving up parental rights alone.