The third stimulus will be reconciled with the Recovery Rebate credit on the 2021 tax return.

IRS wouldn't know about their income being below the $150,000 threshold until they receive the 2021 return.  Since IRS revised the guidance TODAY to exclude unemployment in determining the $150,000 limit, and Intuit hasn't changed the software yet, I assume they don't have this information about your client, right?

Whether they will start kicking EIP #3 out to people once they get qualifying 2021 income tax returns is anybody's guess.  Anybody?