If he took money out after her death, that ought to count toward the 2021 RMD regardless of why he did it or what he did with the money. The problem is he took that money out illegally as he had no legal right to access the account.
It would be nice to tie up this loose end by crediting the amount withdrawn after the date of death to the decedent's RMD under the decedent's SSN but I strongly doubt the custodian would be permitted to report it this way under IRS rules.
Perhaps the custodian could report the after date of death withdrawals as proportionate distributions to the beneficiaries which would then count toward their respective shares of the year of death RMD. The corresponding accounts should then be returned from the estate to the beneficiaries individually. That would make them whole in all respects.
At any rate, I would ask whoever withdrew the money to consult with an attorney as to the proper way to rectify this.