Just a question, if I may. Has your DW tried the tricycle? The reason I ask is years ago, growing up, a neighbor was in the same situation and bought a tricycle so she could exercise. However, she was never able to get any real speed out of it. In fact, most times people could walk faster than she could pedal. Granted, I think a good part of this was due to her lack of physical condition. But I don't think the tricycles are built for any great speed and your DW might have difficulty keeping up with those on bicycles.
I could be wrong, but if she hasn't tried one yet, I'd have her do that before buying. A good bike shop will let you try before you buy.
Just out of curiosity, is there some reason she can't learn how to ride a bicycle now? A bicycle with training wheels would be much better than the trike.
Good Luck, I hope you can find a solution which works for her.
~Rick
2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (27-Angel since 2008), 1 girl (22), 2 boys (23 & 20).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.