[PW] Short story plot

[Kerry Fender]

I have a patron looking for the title of a short story he once read. He
recalls that a man and his ex-girlfriend or ex-wife are on sitting at the
beach. She sits behind him, rubbing suntan lotion on his back and then
absentmindedly scratching his back, maybe with a shell. All too late, the
man realizes the woman has numbed his back with the lotion and has been
cutting his back to shreds.

He thinks it might be Stephen King, but the author certainly owes a debt to
Roald Dahl. Does anyone recall the author or the title of the story?