Dancing With The Chairs (Unabridged‪)‬

Jack is having an affair with Suzanne. He invites her to lunch to tell her that he's leaving his wife so that they can be together. But just before Suzanne arrives, a stranger approaches Jack and asks for help - a stranger whom no one can see but Jack.
Author James Patrick Kelly is a two-time winner of the Hugo Award and a 2007 Nebula Award winner. His stories appear frequently in Asimov's Science Fiction, and he writes the magazine's "On the Net" column. Publishers Weekly has called him "a meticulous craftsman in the demanding short-story form".