"TT prompted me as though it believed that she was required to take an RMD from her own IRA."
No, this question is being asked by TurboTax with respect to all of the IRAs maintained for her benefit, including the inherited IRA, from which she might be required to take an RMD. The total RMD required is the amount from her own IRAs (none required) plus the RMD required from the inherited IRA, which I think you now understand. If any RMD is missed, the penalty is the same and is reported the same regardless of whether she missed taking an RMD from her own IRA or an inherited IRA. There is no need to distinguish between a missed RMD from her own IRA and a missed RMD from the inherited IRA of which she is the beneficiary. An RMD is an RMD no matter what sort of qualified retirement account is involved.
Also note that the page that asks this does say to take into account RMDs from inherited retirement accounts.
