The misspelling of Fiona's name by Fiona herself is a typical and revealing error made by Alzheimer's patients. Coming as it does just after Grant has tried to use the episode of her remembering the recent walk in the park and finding the skunk lilies as a means of continuing his denial, the misspelling brings home to him the futility of his resistance to the truth about her condition.
When Fiona checks into the nursing home, she writes her husband a note telling him to go home. The note is signed 'Fona,' not 'Fiona.'