"The IRS made clear that any late payment of either installment will subject the entire deferral to a late payment penalty. 

• Example: An employer defers $50,000 and deposits $25,000 on December 31, 2021, but fails to make the second deposit by December 31, 2022. The employer is liable for a Section 6656 penalty on the entire $50,000.
• Since the penalty under Section 6656 for failure to timely deposit payroll taxes paid more than 15 days after the due date is 10% of the amount owed, this would result in a 10% penalty of the entire deferral, even though the first installment was timely."

I found a nice article that covers the repayment oversight:

https://www.yeoandyeo.com/resource/guidance-for-repayment-of-deferred-payroll-self-employment-taxes

And remind your client this was Not Income Tax deferral, so it isn't part of the 1040 for the upcoming years. It was reported, and the Payment was deferred.

People get this confused with the covid retirement distribution, where Reporting was deferred.