Used: Craigslist, ebay, flee market, goodwill, freecycle. You get extra points if you ride around on recycling day and pick one out of the garbage, often in near new condition. You might educate yourself on safety issues, search this mesage board. Consider a folding bike with 20" wheels if one is available. I do not know about the hills in your area, but a one or three speed should be fine for her purposes.
If you are handy you can learn how to maintain it on sites like parktools.com and sheldonbrown.com. It is a good idea if she atleast knows about maintaining brakes and tires.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.