A S Byatt 
The Pink Ribbon (Storycuts) 

It has been five years since Madeleine has recognised her husband James. As she drops deeper into her dementia, their lives fill up with the ghosts of her past and of Blitz-era London. When late one night his loneliness causes him to welcome a distressed young lady into their home, he must re-evaluate his perspective.
Part of the Storycuts series, this short story was originally published in the collection Little Black Book of Stories.