From another Father Brown story (The Actor and the Alibi):

The group also contained Ralph Randall, who generally acted elderly character parts, and had a humorous hatchet face, blue with shaving, and discoloured with grease paint. It contained Mandeville's second walking gentleman, carrying on the not yet wholly vanished tradition of Charles's Friend, a dark, curly-haired youth of somewhat Semitic profile bearing the name of Aubrey Vernon.

It is to be feared that the tradition of Charles's Friend has now wholly vanished, so much so that I have no idea what this tradition might be.