Linux

BLUEFIN.

I recently set up Fedora Kinoite on my dad's laptop for him and he seems very happy with it. Kinoite is the atomic/immutable version with KDE Plasma by default. Once I'd set up a couple of things everything else he needs can be installed with flatpak (just make sure to set Flathub as the default and disable the Fedora flatpaks repo that ships broken packages all the time)

Debian is good at being basic, generic, stable AND has an automatic security-update-in-the-background feature

The whole amount of instruction to give to Dear SO is just to reboot the machine if it ever seems to misbehave

If you're not going to give her sudo access then I'd say it'll be really hard maybe even impossible to screw up. Also maybe setup a cron job that'll do auto updates and if needed add in a check to make sure it isn't uninstalling anything. Also how about immutable distro.

I thought this was a request for Stack Overflow proof.

Then figured that was 'proof from pasting random crap from SO".

Then figured it's the same thing.

Any distro will be suitable, create yourself as the first user when installing (which will probably be added to the wheel/sudoers group or whatever) then create a new 'standard' user.

Most distribution defaults should be adequate.

For added safety, choose one that is immutable like, for example, Fedora atomic.

Use btrfs snapshots. Bring the PC to a state that you like, make a snapshot. Then on shutdown set the profile to reload to the specific snapshot.

Any issues? Just restart. Might take a minute, but it ensures the exact same environment every time.