I look at the picture and I'm thinking she may have liked the fit because she liked sitting more upright. If she were to crouch down on that puppy to be more aerodynamic, I'm thinking it would be pretty cramped. But hey, if she likes sitting upright, then let her sit upright.
I'm 5'9", which used to be considered tallish for a woman. My two bikes are both men's 20" Treks and feel fine. Then one ride I was on a rented 20" and was so stretched out my elbows cramped up. I also test rode a 48cm, I think it was. Forget what brand or model. I thought it fit fine, but my sister and other TE friends were saying "No way can that bike fit you!" But you see, I like sitting upright, which a short top tube allows me to do, and as long as the distance and angle from seat to pedals is ok, then the short top tube is not a problem for me.
My conclusion: I agree with the answers above. You've got to find a way to surprise her, yet let her test ride before the purchase is finalized.