There are two different issues here—his mother's RMD for 2021 and his RMD for 2022 and 2023 follow two different sets of rules.
@dmertz , please check me.
1. If his mother was required to take an RMD in 2021 (was past her beginning age) and did not, then he was required to take it for her. Failure to do that would result in a penalty on his 2021 tax return. You will need to check what her RMD would have been, and whether she was drawing enough money in 2021 to cover the RMD or not. If an RMD was required and was not taken, I don't know any way to fix it now, other than file an amended 2021 return and pay the penalty.
2. Then, starting in 2022, your husband is required to take RMDs based on his life expectancy (again, assuming his mother was past her beginning year). He also follows the 10 year rule and must spend out the account within 10 years. However, the regulations for RMDs on inherited IRAs have not been finalized, so the IRS has waived any penalties for not taking the RMD for 2022 and 2023. That means your husband does not have to take an RMD for 2022 or 2023, has nothing to fix for 2022, but should plan to start taking RMDs in 2024.
