Based on my experience this year, I would indicate that the client did receive all of the EIP1 and EIP2 and IRS will correct the return and send what is due to them. I had several clients SWEAR that they did not receive the full amount and all of them got lovely letters and all of them were wrong. I experienced IRS doing a good job. (I know, hard to believe - but true.)
Have your client make their Online account with the IRS; now they can use it to get their Tax Account Transcript, which includes all of the EIP (stimulus advances) for them to provide to you.
Hold them responsible.