Another woman's perspective.
For me comfort is the important thing. My first real bike other than 10 speeds from KMart when I was a kid was a Trek Hybrid about 5 years ago, then I went bent a few years later after my hands and wrists ached for a long time. I'm in my mid 40s. The last time I had ridden a bike was when I was in graduate school in Boston when I was in my twenties. I rode a bike a lot when I was a kid and teen. My sister and I would ride for hours on the weekend when we lived in Chattanooga, TN. We rode all over Missionary Ridge.
If she hasn't ridden a bike in long time and never really developed the balance, it might be hard for her to develop it either on a regular bike or a bent bike. The bent is easier as you can put your feet down quicker and if she hasn't ridden a bike in a long time, then adapting to a bent I think would be easier. But after riding my mom's adult trike while exercising my dogs, it is so easy to ride a three wheeled bike. No fears about falling off or tipping over. My two dogs, 75 and 55 pounds, can't even pull me over. Now they can pull me down the street if I let them.
Comfort, comfort, comfort - would be my goal for her. Comfort on the bike. Comfort in the bike store. Comfort in the bike purchase. Comfort on the bike rides. If you can find a bike shop who is willing to spend time with a novice/newbie then go there. Make sure your wife is comfortable with the store and they are willing to spend some time with her. Maybe make an appointment to go in or ask when a good time for an experienced person at the store to spend some quality time with your wife. If she is not comfortable with the store, then find another one. This is why I frequent the LBS that I do. I went to one store and I felt they ignored me as I was an overweight female and wasn't their idea of a 'real' bike person. I was ready to buy a bike from them, but didn't because of the way they acted towards me. I went to a different bike shop. They took their time with me, didn't make me feel like an idiot, and talked to me about bikes. I knew what I wanted and talked to them about it. They treated me very nicely even though I wasn't a bike racer. When I was ready to buy my trike, they were who I went to. They know my name and when I told them about my accident and my need to get back on a bike, that was their goal to help me to do that.
Then on your first couple of rides, go to a bike path where she doesn't have to worry about cars or traffic and as one person said, ride behind her or beside her. Just go for a cruise down the path. Maybe have a destination for lunch or take a picnic lunch. Bike a mile or so, eat, relax, then head back. Put your bike in a gear that slows you down but so that it doesn't look like you are riding slow for her benefit.
Make it fun for her, not for you. Don't push it. If you can get her a bike that she is comfortable on, then I think the rest will come. The fear is a really hard thing to overcome. Patience and understanding. Slow and easy.
I lost 75 pounds while riding my bike and I hope will lose another 50 when I get serious on the riding and diet.
Christy
Hope this help.