+1 to bringing something from home or the airport that you know she'll like.

There is a Delta food thread, and you can scroll through the recent posts there to see if there are any pictures that reflect the current menu (perhaps seeing a picture of the dish might make you mom feel more comfortable?) Delta In-Flight Meal/Food Service: The Definitive Thread — 2022 Edition

At least before covid, sometimes if you ask nicely, a flight attendant may let you have/purchase a "flight fuel" box which is technically meant for the economy cabin and not first (which is usually a sandwich or cracker & cheese platter), though it seems to be up to the discretion of the FA whether to make an exception or not. (At any rate, I wouldn't rely on this option!)