You report the distribution(s) in the year TAKEN (distributed/received), not in the year they *should* have been taken.

Your client will have three years' worth of RMD's to report in 2023. 

As you are preparing 2021, and 2022 now, complete the part of F 5329 for each year that asks for the waiver of the penalty.  Explain, verbosely, everything your client has done & is doing going forward to make sure future RMD's aren't missed.  

Don't compute the penalty or have your client pay it now; the IRS (in the past...) has been very forgiving on these.