The Masqueraders by Georgette Heyer

The morning's ride sowed promising seeds of a new friendship. The Honourable Charles had an engaging frankness; he kept no secrets from those admitted into the circle of his acquaintance, and it seemed probable that his life might be an open book for Prudence to read if she had a mind that way. With admirable dexterity she steered all talk into channels of her own choosing. She was certainly not squeamish, but half an hour spent in the company of the expansive Mr. Belfort was enough to show that the greater part of his reminiscences was calculated to bring a blush to maiden cheeks. Prudence maintained an even complexion, and had sense enough to think none the worse of him for all his lurid confidences. Sure, they were not meant for a lady's ears.

The ride at an end, it was Charles and Peter with them; they might have been blood brothers. Prudence acquiesced in it, but grimaced to herself when she reflected that it had been in her mind to lie close in London. Evidently this was not to be. But there was nothing to be feared from Mr. Belfort: the disguise was deep enough to hoodwink a dozen such rattlepates.