I worry about this as well as a diabetic. I have longer healing times and I get some nerve issues, usually in my thumbs. My doctor told me if I did all the 'right' things on the long term it would take over a year. He had some math on that I don't recall. It was a slow slow .mm per xxx month. I was pretty hardcore with my routine I did not faulter with sweets or anything. It worked. However, hardcore is never sustainable and as life goes something happend and knocked me off the rails. Time to get back on.
Your doctor telling you it was impossible reminds me of a documentary I once saw about a man that was heavy into meditation and breathing techniques. I think he’s known for his breath holding ability and ability to withstand extreme cold. (I could be wrong it’s been awhile) If I recall he did a long run with no shoes in the arctic or some other cold region. He got severe frostbite and the doctor told him it was impossible to heal. This guy proved him wrong with meditation techniques. Doctor. No explanation. Mind & Body.
Many have pre-diabites, especially in the U.S. the S.A.D (Standard American Diet) is just that, sad. While changing my diet helped me, I'm a data guy and I could easily see a pattern change when missing exercise, especially on an extended period. More than a couple days and my sugar pattern immediatly changed. If you eat poorly, or spluge on sweets on holidays etc you will see a spike. Spike don't matter. it will even out, it's that sustained that will get ya. Treadmill not for me, walking around my area is not for me, A.I. headset has worked well for me to get my daily excersize in without being bored out of my skull, & it's fun, entertaining and can be competitive which I like. My main goto is 'Beatsaber' I shreaded #'s on that and it only requires about a 5' round area I'd say.
When I say exercise I mainly mean just walking. Since your wife is diabetic, you are already well versed in diet. Here, there is diabetes, auto immune and pre diabetes to deal with. I get diet (now) 
