They will need to report the $2800 the IRS sent them. Most likely they received it on a debit card and cut it up thinking it was some kind of scam (like some of my clients did).

They can file form 3911 (separate from the return) and request the IRS to trace the payment and re-issue.

As to the amount of the credit - the advance credit was an estimated amount based on the prior return filed. The 2021 return is a "reconciling return" which allows you MORE credit if you are entitled to it based on your 2021 return, but fortunately did not require a repayment if your income increased in 2021.