Well, if you're drafting all the way, you might just be able to pull this off on a beach cruiser.

Anyway, I don't think mom can keep up, no matter what bike she gets. Even if she gets a fast road bike, she will not be able to ride as fast as her husband since he is an experienced fit cyclist and she is just starting out. I think what we are trying to do here is find a way to reduce speed differential to minimal possible under the circumstances. On the actual ride the husband will still have to slow down (esp. if mom isn't comfortable with drafting) if they want to be riding together.