I say don't do it. Was recently freelancing in an office where in the middle of my gig they upgraded to Yosemite to make it play nice with their server. Noticeable difference. Flickering when moving the mouse, unexplained hang ups. And this was on a relatively new iMac (thin version) with 16 GB of RAM.
If she's that light a user, I think you're rolling the dice. Not saying it's going to wreck the machine, but I wouldn't take the chance.