In a matter of months, my mom has gone from living at home showing signs of mild dementia to living in a nursing home to a doctor recommending we put her in hospice for advanced dementia. We had to put her in a nursing home because she was an invalid and my dad with Alzheimer's could not longer take care of her or himself.
Can dementia progress that rapidly?
"Couldn't we talk about something more pleasant?" by Roz Chast.
I walked around with this book clutched to my chest for about five months when my mom was in the worst of it. The book is helpful and very funny. It describes this situation to a T.
It may also be, as in many cases, that your mom was already quite badly off but it was masked by having your father run interference and take over on all fronts. she also had the "help."
This is so common that sometimes family members who visit routinely do not realize how bad things have gotten.
Be grateful that she was home and "functional" for so long.
Good luck!