Thanks for your thoughts Iride and Mitch...
We have ridden together and I just adjust to her pace, that part is fine I'm happy to have her along. A comfortable upright ride is what I'm thinking - we bought a MTB for her off craigslist thinking it would be good for occasional trails and gravel. In the end she never rides trails and its too klunky to be very nice for the road which I'm enjoying more and more. Its a low-mid range bike so its heavy and i don't mind spending a bit of money to get her something light, stable, comfortable and more road worthy.
She'll do a 10 mile country road ride with me, so while the beach cruiser and fat tire might be comfy, I'm thinking a touring bike, hybrid or cyclocross will be my best bet, good to have the input of those who have owned one of those, I never have.
And Mitch, I appreciate the gift understanding - even if she doesn't ride it often, she will time to time, and trying matters! Basically I'm looking to exchange her MTB for something better for her on the road.