I want to relate a story about fasting. I had been telling my elderly mother about fasting and its benefits for a few weeks. She has stated how concerned she is about her diet. With her dementia she doesn't remember much of what I say but it does somehow seem to make an impression. She told me that she was depressed and had decided to stop eating and just let nature take it's course so to speak.
Her plan failed miserably. After two days of fasting she felt better. Her depression and anxiety cleared and her mental clarity increased. She's now back to eating.
You see fasting causes a person to go into a state that's called autophagy. In this stage the body kills off disease and removes it from the body. It's basically the process by which cells maintain their integrity and function by removing damaged or unnecessary cellular components. It can't cure everything all at once but it does more for a person than you would think.
If you want to know more about autophagy look up Nobel Prize in Physiology or Medicine in 2016 awarded to Yoshinori Ohsumi, a Japanese cell biologist, for his discoveries related to autophagy.
I'm very sorry for your mother--and everyone who loves her. The deliberate cessation of food/drink is recommended by the "Compassionate Care In Dying" group because they realize the medical community will reject anything else. Your experience demonstrates why it's not a good solution to the problem of wanting to end one's life. Wishing her a quick resolution.