their 2021 income will limit the amount, but you have a bigger issue...

If IRS sent them the 6475 letters that they DID get it, (most peoples were based on their 2019 income, not 2020) and they insist they didn't, you should be filing a Form 3911 to trace it, DO NOT put on the return that they didn't get it, IRS will adjust the account.

This is actually better for them.   If they can get that 2800 advance reissued to them (assuming they really didnt get it, but Im betting its in their bank statement for March 2021), they make out better than getting the 700 allowed with the 2021 income.