Make her install Gentoo in her own and make sure to berate her at every step /s
Personally I would go with something that has KDE so she gets fancy animations and stuff, but Mint is a perfectly valid choice too (cinnamon also has some cool stuff to toy around with but not as much imo). Just make sure to show her how to install stuff, and put adblockers and shit in the browser. Also highly recommend installing some of the linux games.