You need to knew her leg length (from the floor to the pubic bone). Many shorter women have very long legs. I once had a girlfriend who was four inches shorter that I am, yet my 32 inseam jeans fit her like a glove...she had the same 34 1/2 leg length that I have.

Most women's road bikes have compact frames, which use goofy sizes based on their very short seat tubes. A more important measurement to know is the standover at the highest portion of the top tube (just one inch behind the stem).

Measure the distance from the highest portion of the top tube straight to the floor. That distance should be about the same as her slack's inseam. So, on my road bikes, the highest portion of the top tube is 32 inches or 33 inches from the floor, compared with my slack's inseam of 32 inches, and my 34 1/2 inch leg length.

If the highest portion of the top tube is about 1 1/2 inches less than total leg length, the rider can safely stand over any portion of the top tube, but the frame is tall enough to enable the rider to raise the bars to a comfortable height.

Many/most females are most comfortable when the bars are at the same height as the saddle, or about one inch lower than the top of the saddle. That higher bar position permits riding with the pelvis upright, and the rider's weight on the "sitbones" and not on the soft bits. Obtaining that higher bar postion requires riding the tallest frame she can stand over.

Likewise, most female riders do not like to be overly stretched out, with too much of their weight on their hands. A taller frame raises the bars and brings the bars back closer to the saddle (and that is still true even if the top tube is slightly longer on the taller frame).

Make sure that the bike shop understands that AFTER you give her the bike, she will be bringing the bike in for a fitting, and possibly exchanging it for a different size/color/model. If a shop won't agree to a "free" exchange, buy from a different shop.