My father passed away over 3 years ago For the last year of his life my 90+ mother and myself took care of him. He was also in a rehab/nursing home for about 8 weeks. My mother recalls that time and the circumstances of his death incorrectly. She seems to want to blame his death on anything other than it was his time. I don't even try to set her straight as it has led to her getting angry at me. I'm tired of hearing about things that never happened or people who were to blame. My father lived a long, mostly healthy life and I am grateful for the time I had with him.
I think part of it was her dementia, but a bigger part was the complete and utter shock of losing him. I don't think she ever thought he'd go first.
There's no use trying to correct her or argue with her as you will never win and only aggravate her and yourself.
So just go along with whatever she is saying and move on. It's really not a hill to die on.