In the novel Curtain, Mrs Bradley was Derek Bradley's wife. When she discovered that Derek was having an affair with a much younger woman, she threatened to kill him. Derek later died after drinking a cyanide laced beer. Mrs Bradley broke down and confessed that she had killed her husband. She was convicted and hanged.
It was later revealed that Mrs Bradley had killed her husband without realising it, as she was mentally "controlled" by Stephen Norton (Norton likely remarked things that made the victim go insane and commit murder).
Arthur Hastings told Poirot that he remembered the Bradley case, and that Mrs Bradley was a very good-looking woman.