Visit a few bike shops, and ride some different sizes to help you determine what size she needs, and likes best.

Then, go buy from whomever fits your budget - nothing wrong with buying on-line....... To me, many sellers, including LBS, often try to put you on a too small bike.

The reach will be the deciding factor, as it soundls like she has long legs for her height. Can she reach the handlebars comfortably, is she cramped, does she feel like she is too stretched out? Seat tubes are ultra adjustable, reach - not so much.....

Buy what you can afford, and enjoy the wind in your face OFTEN!