

Mint is good. Fedora & KDE might be a good choice because of the customization. Could be a nice way for her to make her desktop her very own creative project that inspires her to learn more about her computer and how it works. Fedora is pretty damn snappy. dnf is really easy and she could just use discover or whatnot to install apps.
Help her rice up her desktop a bit and show her how to install some programs. Maybe bookmark some tutorials for different programs you install and the OS of your choice.
Blender, kdenlive, krita, gimp, inkscape, strawberry for music, libreoffice, vlc, supertuxcart etc…
Then turn her loose and let her have fun.
Whats the square root of negative one?