Bah, I'm like 6' 0.5" tall and ride a medium/16. I muchmuchmuch prefer the increased agility and climbing it provides me over an 18 or 19 that I "should" be riding. I've been seeing people accept down-sizing more and more lately. I moved my saddle back a little extra, use a longish stem that's lowered just a little extra, and use 180mm crank. I'm almost too fast in technical, twisty singletrack now--when I raced, I'd find myself forced to slow-down and wait because you can't really pass in those situations. Though in some areas where the boundaries were a bit wider, I'd get too impatient and pass by simply riding clear off the trail to get around.
If you want it, ask her if you can experiment with it a little before selling it to see if you can get it to work for you or not. Nothing to lose but time, right?
Judy DH--80 mm
of travel! WHOOOAOAAOAOOO!