Keto diet may reduce Alzheimer's risk by altering gut fungi:
https://www.medicalnewstoday.com/articles/keto-diet-may-reduce-alzheimers-risk-by-altering-gut-fungi#Fungal-signature-and-diet
Does a low carb diet keep your brain young?
https://www.medicalnewstoday.com/articles/does-a-low-carb-diet-keep-your-brain-young
In reality, most of the “benefits” of Keto also come from just a “lower carb” approach. Getting blood sugar and insulin to both work again in their normal manner is the key. That can also be done with constant exercise to burn off the carbs. Stop eating breads, sugars, and starches in favor of the most complex of carbs, found through Glycemic Index/Load tables. Limit artificial sweeteners, which were found to cause insulin release even in the absence of calories (for that matter, it's not clear natural sweeteners like monk fruit or stevia don't cause the same reaction).
There are supplements to help, but the biggest initial benefit is from diet and exercise changes getting rid of one's “fatty liver.”
She could see immediate improvements and limit any progression.
Totally agree with what you are saying.
If you want a reference, check out Amy Berger. While she is primarily focused on Alzheimers, the discussion of a low carb, high fat diet should be helpful. Also, since patients often have a combination of dementia and Alzheimers, the info could be very applicable. The ketones that come about on a low carb, high fat diet are very good for brain function.
https://lowcarbmd.com/episode-11-amy-berger
Amy’s book: https://www.amazon.com/Alzheimers-Antidote-Low-Carb-High-Fat-Cognitive/dp/1603587098/