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:
- KDE - Very configurable, nice looking, a bit heavy.
- Gnome - Simple and very opinionated, so not very configurable, a bit heavy.
- Cinnamon - Should feel familiar to Windows users, a bit faster than KDE and Gnome.
- Cosmic - A middle ground between Gnome's simplicity and KDE's configurability, pretty fast.
- XFCE - Very fast and light-weight, fairly configurable, but not very flashy.
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:
- Fedora or Ubuntu with Gnome
- OpenSUSE with KDE
- Linux Mint with Cinnamon
- Pop!_OS with Cosmic
- Mint or Ubuntu with XFCE
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.