I would write a letter to the Credit Union describing that despite all evidence that shows otherwise, which you brough to their attention on more than one occasion, they seem to leave funds on deposit, which do not belong to this other person. They seem satisfied that is should be considered a legitimate deposit to their account, which you feel does not meet their requirements under the Patriot Act to hold the financial institution responsible that money is not being placed in accounts without proof of identification for the funds sent to the receiver, since that bypasses their responsibility to avoid or prevent money laundering or support of terrorism. You have done your best to bring to their attention that the wrong party has these funds, which makes it available to an unintended party to do whatever they want with the funds, and that you certainly can bring up the lack of identity confirmation and their lack of cooperation, to the IRS, when the funds are in fact a Federal Payment from the IRS payment directly to the party holding the funds in error and clearly meant for your client, that actual tax payer. You can make it clear the IRS can be brought in to examine the banking flow.
"Level Up" is a gaming function, not a real life function.