I think it'll be best for her to see a geriatric psychiatrist or geriatrician. A MMSE or MOCA test will help with diagnosis. She'll need someone who spends time with her to go for the appointment with her. They'll be able to give details she can't remember that'll be usefull in diagnosis. Things like how she does with activities of daily living, finances, directions, learning new things, sleep/wake cycle etc.
How is her general health? A medical illness even a urinary infection can set things off but it's been going on a while so sounds more like a cognitive decline. A rule out of depression would be usefull too.
Occassionally a neuropsychological test, MRI or PET scan is done but these are mainly academic and not necessary for diagnosis usually.