We would assume the first EIP was paid out before the 2019 tax filing got processed in 2020, showing the date of death for the spouse was in 2019. And it is possible the second payout didn't happen at all, taking the first payout into consideration.

He hasn't returned that check, which is what the IRS announced should be done back then, for people not alive in 2020.

The IRS has info for when an EIP might be "garnished" and I have never seen that one; the wording is different for each EIP.

 Have you filled in the 1040 showing that for 2020, he got more than he is entitled to? The 2020 tax form is supposed to be the reconciliation.