My very first thought about this is: Has she expressed an interest in starting to ride a bicycle? Has she mentioned to you, or to her other friends, that she wants to . . . commute to work? Start exercising to lose a few pounds? Ride with you? Or some other reason for her to have a bicycle?
If not, and if you present her with a bicycle of any sort, she could have a very negative reaction, the first part of which would probably be: "Do you all think I'm fat (and need to exercise)?" ... followed by something along the lines of: "Only children ride bicycles! (do you all think I'm that immature)"
Sadly, bicycles are still viewed as tools of exercise torture, or children's toys by the majority of the population.
However, if she has expressed an interest in cycling, I agree with what ken cummings has said. Make sure it looks great, and is in excellent working condition. Make sure it is not something you would be embarrassed to be seen in public with. As a new rider, if she is going to be motivated to ride the bicycle, she has to feel comfortable on it ... and comfort goes beyond a good fit, it also encompasses her perception of how the bicycle looks and works.
As for fit, are their any of her friends who are roughly her size? Is there any way you can get her inseam measurement? If it is a road bicycle, she should be able to straddle it (standing flat footed over the bar) with at least an inch to spare. If it is a mountain bike, there should be a couple inches. Do not give her a bicycle she cannot straddle ... a bicycle that is too big makes new cyclists very nervous.
She should also be able to reach the handlebars comfortably without too much of a stretch. It is a good idea to have the handlebars about even with the saddle, or maybe even a touch higher than the saddle.
And you might mention that you would be willing to change the saddle for her if it is painful. To start though, make sure it is not the narrowest saddle on the market, and also make sure it is not one of those wide tractor-seat-like saddles. Think ... mid-sized! And then work from there.
I hope some of this helps.