The answer depends on how the money was moved to your mother's IRA. If it was done by reportable distribution and rollover (not the preferred method), the money distributed to your mother included your father's 2023 RMD and the rollover of that portion constitutes an excess contribution to your mother's IRA to the extent that it exceeds the amount your mother is eligible to contribute to a traditional IRA for 2023 as a regular contribution. As an excess contribution, the amount would have to be removed from you mother's IRA by a return of contribution.
If instead the money was moved to your mother's IRA by nonreportable trustee-to-trustee transfer (the more proper way to move it), your mother simply needs to take your father's 2023 RMD from her account. In this case, the IRS does not care that the RMD was taken after the transfer and requires no special explanation except in the unlikely event that the IRS questions whether or not your father's 2023 RMD was completed. The 2023 Form 1099-R will show the total of the regular distributions taken from your mother's IRA.
