Usually it is good to exercise. I cannot say for her because I don't know if her health allows her to exercise or not, but I can explain a little bit about how walking works for dementia elders.
My father-in-law has late Alzheimer's. This is a slow process. Long before late 2006, my FIL was able to jog for a long distance all the way to the beach and campus that is 30 minutes or an hour away from his old house everyday. Starting in 2007, he could no longer jog but he tried to walk a block. I think he had a cane already in 2007 but he used it only sometimes as he tried to be normal without a cane. Starting in 2008 after he got sick once and his cognition level dropped a little bit, he needed a cane for sure. He could not use a walker as he cannot drive anyway. A walker is harder because it needs the drive which he can no longer understand how. Now it is 2010. He fell once or twice so someone has to be next to him this year. He still uses his cane and it is better if someone is next to him to make sure he does not fall - the difference is he uses it all the time for walking. It is a very slow process and it took 4 years! He still does not need any wheelchair at all.
So if your MIL can exercise if the doctor allows it, it is OK. Yes she would forget how to walk but it is not a drastic change. It is a very slow change. So you can certainly give a trial to see how she can walk. But a walker is not a good idea if she does not drive anymore. A cane is good. Make sure someone is next to her.
If the doctor allows it, some little exercise or even physical therapy is good.
I don't think having dementia means no exercise anymore. In fact, it is the other way around: more walk or exercise helps. The director of the home where my FIL is encourages him to walk more without a wheelchair.
Hope this helps,
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