Even the medium (15.5") frame is going to be long for her with its 23" top tube, though it has a low standover. The Large, with its 24" top tube, would be spot on for you, but it'd be like trying to ride a bike about 3 sizes too big for her! Despite the low standover relatively… With a 10" height difference, and women generally having shorter torsos and longer legs than men, unless you're happy riding a bike 2 or 3 sizes too small for you, or her one 2 or 3 sizes too large, you're just going to have to buy her her own bike I'm afraid!