Defense against Child Support Non-payment claims

To whom it may concern
My brother in law is in trouble with child support services. During
1996 - 2001 he paid child support to his ex. Directly, outside of
child support services. Now child support services is saying they do
not have records of him paying and now owes approx fifty thousand.
Because this period of time was years ago he no longer has proof that
he made these payments. He is getting the run around by a caseworker
at Child Support. He has told him he needs his ex to send in a signed
letter stating he made the payments during that time. My friend then
asked his ex to do this for him. To make a long story short, now she
wants approx $2400 in order to write the letter for him. My concern
would be, even if he does pay that doesn't mean she's going to write
the letter.

I have heard that he can send a "Request for Admission" to his ex
trying to get her to admit that he did in fact pay the child support.
So, I did some of my own research and I'm wondering whether he can
send a "Request for Admission" when there is as of yet no case
pending.
He can possibly go to jail because he owes so much. Now, because they
are going to garnish his wages he can lose his house. He's current
family is suffering. Is there any advice anyone out there can
offer?

Thanks