Ditto on the STAy AWAy from aluminum frames, even with a carbon fork if your mom is looking for a road bike. I am on the "tall side of short" and am in the process of buying a steal framed bike (a colnago). I heard great things from Terry riders-they love the ride of their bikes. Also, Bianchi makes a smaller framed road bike that uses 650cc wheels on both front and back and it is a steel frame with drop handle bars with reach developed for women.
If your mom doesn't want a road bike but a mtn bike type frame, steel framed. Like penny said: Kona makes a great frame. My mtn bike is a Kona, steel frame.
The best thing is for your mom to test ride as much as she needs to. A good bike shop will spend as much time as she wants/needs to feel comfortable.
Keep us posted on how it goes for your mom!

stella