Dr Ridgeway was a doctor who had a practice near Poirot's lodgings at 14 Farraway Street. In the short story The Unexpected Guest (chapter one of The Big Four), he is summoned to examine a stranger who has climbed into Poirot's bedroom but who does not speak except to repeat mechanically the phrase "M. Hercule Poirot, 14 Farraway Street".
Later, in The Terrible Catastrophe, he treats Hastings who is injured in an explosion. He tells Hastings that Poirot died in the explosion. In The Dying Chinaman, Ridgeway advises Hastings, who has recovered for his injuries, to drop the Big Four case and return to South America.