I actually did email some woman the last guy I lived with was talking to just before he moved out (technically not unfaithfulness--I'd already told him I wanted to split up), telling her that he was a liar.
It made me feel creepy and stalker-ish and I never did such a thing again. I was angry at him in general and drinking and used lousy judgment. It wasn't my business, and honestly, my motives were at least as much to get a dig in against him as it was altruism to warn someone else.