From what you describe (an incision almost all the way across her hairline) it sounds as if she had a craniotomy.
I had one two years ago. Afterward I had some trouble retrieving a particular word during conversations. I asked my neurosurgeon about this and he said it was normal, from the anesthesia. And he said it would resolve in the next few months, which it did. ( It took about 3 and a half months.) For older people the anesthesia can be really hard. My father-in-law had surgery and afterward kept asking where his wife was. (She had passed away years ago.) If she is not better in a month I would insist on another consult. Her condition sounds more extreme than what I experienced and I wouldn't wait 3 months to see if she got better. Perhaps the neurosurgeon would pass her on to a neurologist who specializes in dementia issues.