I watched my grandmother make a change very much like that. After her husband's death and the beginning of dementia, family moved her from assisted living in Washington, DC where she'd lived for 50 years, had many friends, an active social life, and of course access to public transportation and infinitely varied cultural opportunities - to my uncle's home in literally the middle of nowhere, 30 miles from the nearest convenient store, where she knew no one, and -40° winters to top it off. It wasn't good for her. She deteriorated quickly. Would she have kept more of her mind longer if she'd stayed closer to home? No way to know of course, but most of the research suggests she could have.

If your friend has already made the choice to live beyond the point where she has to be dependent on others, the choice then becomes whether she wants to depend on one person, her son, for EVERYTHING, or whether she has enough financial resources to hire caretakers and/or a living facility of her own choosing and stay within reach of the friends she's made over the years. Unfortunately, for many people, finances dictate the choice. I hope your friend at least has enough funds to feel that she has a choice. It's much easier in Canada than it is in the United States where health care costs are such an enormous proportion of household expenses.