Old minor disability payment dispute

Hello! I’m new to reddit but I don’t know where else to look for help, so I’m here.

One of my friends is in his 30s, and when he was a minor he was disabled and received social security payments that his mother deposited into a college fund account. Due to an administrative error they had underpaid him, and owed him $2k. Instead of that they accidentally sent $15k, but he had already graduated college and the account had been closed. So, he never received the $2k he was owed, much less the $13k they mistakenly sent. When they tried to get him to pay it back, it was established with a case worker that the payment never deposited, and he went on his way.

Last week he received a letter demanding he pay them $15k. He told them this had already been settled years ago, but they are now making him fill out a massive form to prove he is poor/disabled instead of simply checking the transaction history associated with the account.

All of this is to say… not only is he experiencing distress by having an administrative error being blamed on him, but he never received the $2k he was owed in the first place.

Any suggestions on what he can do? In addition to having them get off his back for the extra money he never received, I’d ideally like to find out if there is a way to get the payment he is still entitled to.

Thanks in advance!