If you want to replace that Schwinn: These look fantastic: http://www.bikesdirect.com/products/city_bikes.htm Something with the internal gears will be less maintenance, chain cases so she doesn't get gunk on her clothes, loop frame/ mixte for easy-on easy-off. Prices look pretty decent.

If she is wanting to keep the Schwinn for commute duties and add something more road-ish, then possibly look into a nice older mixte on craigslist. If you can find a nice CroMo frame, it won't weigh a ton, and should be relatively cheap. Depending on what you end up with, she can either ride it as-is and replace parts as they break, or you can spruce it up right from the start and she will have a nice "fast" bike to ride with you. It would be something you two could work on together.