"The credit union sent a 2025 1099-R showing the combined RMD amount taken from “her” IRA and the one she got from my Dad. The Box 7 code is “7.”"
This implies that there is only one IRA, which is fine. The amount that she was required to take was the sum of her 2025 RMD and the portion of your father's 2024 RMD that your father had not satisfied before death. The completion of your father's 2024 RMD is not treated as being late because if a decedent's RMD is completed by a beneficiary by the end of the year following the year of death, it qualifies for an automatic waiver of the excess-accumulation penalty.
"do I check the box that says, “I inherited this IRA?” Or because the IRA was opened in my Mom’s name in 2024, the 1099-R is under only her name, and Box 7 uses Code 7, is this IRA fully considered HERS, and I just check the box that says none of this applies to her?"
The latter. The IRA is her own IRA, no longer treated as an inherited IRA.
