Have you actually seen their 2020 tax return? Is it possible that she was also claimed as a dependent of his in 2020?

Also - What did he ACTUALLY receive (ie. what got deposited to the bank if he had direct deposit)?

Another note. Even though the IRS was (I believe) authorized by Congress to put in place anti-abuse rules for the EIP3, noting so far indicates this is the case, and on my returns Pro-Series has not been computing a payback where the amount allowed per the 2021 return is less than received.

You CANNOT split his 6475 letter between two separate taxpayers. The EIP follows the actual recipient not the taxpayer.