As others have said, there are varying opinions on exercise.
I had one doctor (general practitioner) tell me "use it or lose it". I think that was bad advice and I no longer see that doctor.
I have been using a wheelchair for 2 years. I use a wheelchair quite a bit, even though I am still able to walk within my house and can climb a flight of stairs (very carefully!). But, walking to the end of my driveway wears me out. Walking farther than that is no longer practical for me.
By using my wheelchair as much as I do, I believe that I am prolonging my ability to do important things, such as unassisted toileting and going on trips to my favorite national parks. So far, this has worked well for me.
I do still ride a bike on a stationary trainer, though I can only stay on it for 10 minutes and cannot ride it every day. I always push a very easy gear, basically just spinning.
I think everyone is different in their response to exercise. A bit of trial and error may be necessary to figure out what works. If your fiance does experiment with exercising, I encourage going very slow until you know what helps and what harms.
Several replies brought up the topic of falls and how that can seem to hasten progression. Early use of a wheelchair is probably the single best way to prevent falls.
Falls are insidious. I am intellectually aware of my risk of falling. However, when I am doing things, I don't recognize that a fall may be starting. Once it starts, I don't have the ability to stop the fall. I just go down...often hard.
I have broken a toe, sprained an ankle, and broken an elbow in various falls. Hopefully, your fiance can learn from my folly! Once I recognized that I really was at serious risk of being injured in a fall, I started using my wheelchair more. Since then, I have been battered and bruised, but have not suffered a serious injury in a fall.
Steve