Most of my clients got it via Direct Deposit.  So they can look up their bank statements to find how much they received.

The first link given above is where the client can potentially set up an account to pull up their 2020 "Tax Account Transcript" which would show the amounts.

If they are having problems doing that online, the client may be able to fill out Form 4505-T and mail that to request the IRS to mail them the 2020 Tax Account Transcript.  But that could take several weeks to several months, so they will want to file an extension on their tax return.

Or as mentioned above, tell them if they can't give you a definite number that you will assume they received the full amount.