I can't see an 85 year old being comfortable getting on and off a low recumbant bike. It's enough trouble getting on and off a couch!

I think you should get your mother a comfortable mixte, and then get some adult training wheels on it for balance. There are several training wheels for adult bikes on the market.
Here is a start:
http://images.google.com/images?hl=e...-8&sa=N&tab=wi