A book about a man on a planet of rat people

My coworker is trying to find a book she read as teenager in the 1970s. She cannot recall many details at this point aside from the fact that the protagonist is a human man among rat-men with rat faces, perhaps on an alien planet full of rat-men. She believes that the protagonist was the only human character in the novel. It was probably written at some point in the 50s or 60s. Does anyone have any idea of what this might have been?