Going 'telling mode'.
it's not certain and it's not a one way trip.
Her life has been devastated, she went through hell, she lost her husband, she's moved.
Every possible additional cause for confusion and uncontrolled stress is there.
That she forgot why she was sad is a bad sign but it sounds like she needs positive actions in her life and not grief after grief, she may well stabilise well even if she does have early Alzheimer's.