Her workflow most likely just browsing, email, FaceTime, reading, thats all.
As well as being more familiar right away because it will mostly feel like a bigger iPhone, it's a better device for reading (put the Kindle app on there with a Kindle Unlimited subscription for a bajillion books and some free magazine subscriptions), it's got a better camera for FaceTime, if she's at all artistically minded there are a ton of excellent drawing apps to go with the Apple Pencil, etc.
The one big caveat I would point out is that the gesture controls have really poor discovery, so you may want to print out and laminate a cheat sheet (like the illustrated versions on https://ipadpilotnews.com/2020/04/all-the-gestures-ipad-pilo...) for easy reference.
My advice to future eldercare providers is to pick a tech stack, lock down those skills, and then stick to it.
The current iPad, iPad Air, and iPad Pros all work with some version of the Pencil. Any recent iPad will have 1080p FaceTime rather than 720p of the MacBook Air. This new iPad Pro has even higher resolution with automatic panning that could be useful if they want to set up the iPad on the kitchen counter and walk around while they talk to you.
As for being foolproof, it is about as foolproof as an iPhone is. So if you count iPhones as foolproof, then you should be good to go.
MacOS, for all its good, is an entirely separate beast, and she will essentially have to learn a new OS, with all the caveats that come with it, not even mentioning foolproofness and such.
For your specific scenario, sounds like iPad Pro is the better of the two options.
I do not see why a non tech literate person would need an iPad Pro though. iPad Air (bigger screen is more comfortable for older eyes) or even Mini should easily suffice.