The Bombe machines used to decipher Enigma are usually described as being electro-mechanical devices. Perhaps you could argue they were special-purpose computers of a sort, but definitely not the programmable, general-purpose machines most people think of when they hear the word "computer".
The first programmable electronic computer was developed at Bletchley Park, but it wasn't used to decipher Enigma.
Any of the large, easy to use distributions should work just fine. I'd recommend a popular distribution because it'll be easier to get help online. So consider Mint, Fedora, OpenSuse, Ubuntu and maybe Pop!_OS.
I think the main consideration should be which DE (desktop environment) she'd like to use. IMO the main contenders would be:
Based on which DE she prefers, I'd suggest getting a distribution that comes with said DE by default, for the best possible integration. How do you figure out which DE she likes best? Put Ventoy on a USB stick along with a few different Linux ISOs. Ventoy wil let you choose which one to boot from a menu. You could get the following ISOs:
Download an ISO for each, install Ventoy on a USB stick and copy the ISOs to the stick. Boot into each ISO and play around with the desktop for a bit. When she's figured out which DE she prefers, install a distribution that comes with that desktop.