I think that this story is in the Brother’s Karamazov. It’s an old Russian folk tale of a woman who does one good thing in her life by giving a poor person an onion to put in his soup and after she dies as she is flying towards hell, she sees that onion and grabs it and hanging on to it escapes hell and heads towards Purgatory instead. C.S. Lewis would recognize this man as one who had some much going against him and his one good act is so much more full of grace than the best thing any of us normal people with good constitutions and education etc could ever hope for.
Well said.