I'd say go ahead with Mint, MATE edition is my preferred flavor.
Set it up with the Compiz 3D desktop cube switcher thing, and maybe a couple other nifty effects to mesmerize her even more!
Sure she'll likely end up tinkering more with the Compiz settings and bork the settings a bit after she sees what all it can do, but it can totally be a fun learning experience, and Linux is easy enough to reinstall anyways in worst case..