I'd get her an upright comfort bike to try out. Mountain Bikes and Road Bikes have aggressive geometry and can be uncomfortable to some people.

I'd get her a cheap one from Craigslist (make sure it's in good order, or fix it up if it isnt). Or, better yet, RENT her a bike. See if there are any bike shops that rent out bikes in your area. You can get her a top of the line comfort bike for about 30 dollars for the day.

Here's my local rental spot: http://bikemart.com/articles/rental-services-pg152.htm