She is 90 years old, and just plain mean. Refuses any assistance from other than son. She has become agressive, angry, accusatory, unreasonable, unappreciative, and more. I've discussed this with a very dear friend, she mentioned mini strokes or maybe dementia. Could this be a likely cause for her behavior?
She needs to be seen by a geriatrics doctor who can start the diagnostic process, refer her to a geriatric psychiatrist for meds for her agitation and aggression, perhaps a geriatric neurologist for diagnosis.
In any event, son needs to decide what he can and cannot do without losing his livelihood and his life.
Just because his mother wants something doesn't mean it has to happen.
I'd work on having her properly diagnosed. Sometimes, medication can help.
Have your Mom checked for a Urinary Tract Infection [UTI] as that can cause the very same symptoms you had mentioned. The test is easy, and the cure is antibiotics. Take Mom to her primary doctor or go to a urgent care center where they can run the test and get the results in a few minutes.
Has she always had a 'mean' side or this new and inexplicable behavior? As seniors age, they often just become more profoundly "who they really are". Kind people get sweeter and sweeter, mean people get meaner, or don't bother to hide their "meanness" with covering behaviors.
I'd check for the UTI, but not expect that to be the main causation.
Mother now has dementia. She is losing her "filter" and says the most atrocious and embarrassing things. I take her places and just want to melt into the floor sometimes. She is just no longer waiting until we are "alone" to comment on people's appearances, etc. Just blurts it all out. I have had to flat out tell some people, after she's said something completely awful "I'm so sorry, Mother has dementia. She doesn't know what she's saying".
Mother has always been all the things you mentioned. But she kept it "under wraps" pretty well. Now, not so. It won't get better, so, hang on for the ride.
Hopefully it IS as simple as a UTI---although Mother gets those every month and they don't alter her behavior at all.
Is she living with you?? For your sake--I hope not.