At the very least your client needs to make the calculation of the ERC that will be claimed on the 2020 941-Xs.  That amount should be booked in 2020 as a reduction in payroll and a receivable.  I have to ask, however, given the lucrative nature of the refundable credit WHAT IS YOUR CLIENT'S PROBLEM!!!  I have done several 2020 941-Xs, coordinated with PPP loan forgiveness applications (making sure not to double dip on the wages), and they are no big deal.  

PS  It is 6/26/21 and we still have not received refunds or IRS communications regarding the 2020 941-Xs filed MANY months ago.  My prediction, the IRS will never fully recover from this fiasco.  After all, they couldn't even keep their heads above water pre-COVID.