Originally Posted by StephenH

There are recumbent tandems as well, which might take the weight off her arms. There are combined tandems, where one person is in a recumbent seat and one in a normal bike position, but I'm not sure who does the controls on those (I think the normal bike seat is the captain's seat, though in the rear).

Some of the older tandems are made for more upright riding, but they're generally the single-speed around-the-neighborhood type bikes.

There are bicycles that are auto-shifting. They're not too popular, but an option to consider.

On the coaster brake, it depends a lot on the riding you're doing. I'm riding a cruiser-style bike with coaster brake. Works great. But I generally top out at about 17 mph when pedaling, and don't have any long downhills on the routes I'm doing, and I'm not dodging big-city traffic. If you're going faster, in busy traffic, or have long downhills, it could be a problem. A lighter person should have less problem than a heavier person when using coaster brakes.

It occurs to me also that with normal bicycling, you mostly don't use your hands for much of anything but steering or leaning on. But, every once in a while, you'll need to apply some force to the handlebars, and it could be very awkward if you couldn't. Comes up when accelerating hard, for example, or when hopping a bike back onto the asphalt.