If they received more than the letter states, there's no payback or penalties on the EIP payments so I wouldn't be worried about it in that instance.

If they received less, according to the letter, but they actually received the correct amount, I wouldn't worry about that either.

If you claimed the RRC for the 3rd EIP on the TY2021 return, the IRS will be verifying all of those and adjusting the return accordingly. The only issue there is, if the IRS is reporting an amount on the 6475 that is more or equal to the amount you claimed for the RRC. In that case, you wait till the return processes and the Taxpayer receives the refund, if any, before responding to a notice of a change to the tax return, which the Taxpayer should receive. If the IRS does not send a notice, just reduces the refund, then you can amend the return with the bank statement showing the EIP deposit.