Alice Munro 
The Bear Came Over The Mountain (Storycuts) 

A reluctant decision is made to put Fiona in a home catering for those suffering from Alzheimer”s. When her husband, Grant, comes to see her, he is confused and dismayed to learn that she has struck up a close relationship with a fellow patient.
Part of the Storycuts series, this short story was previously published in the collection Hateship, Friendship, Courtship, Loveship, Marriage.