Then the distro doesn’t matter that much. You shouk go for either Snaps or Flatpaks for applications as they won’t break the system even if something catastrophic happens.

Personally I’d go for distros with great deal of support (Pop, Fedora and Mint) and put Flathub as the main source in the respective app stores. Smaller distros tend to have more issues that requires some troubleshooting at times.

Check how does she uses Ubuntu first though, if she’s using the Canonical’s additions to Gnome a lot, she’ll have a horrible time with vanilla Gnome as it’s pretty barebones, that’ll rule out any distro with vanilla Gnome such as Fedora. Trust me, you don’t want to be babysitting your partners computer.