Do you know of any instances where the "other man" or "other woman" contacted the spouse or significant other and told them their spouse has been unfaithful/dishonest?

i was just watching a show where the other woman had been seeing her ex, who was now with a new girlfriend. he had been stringing the ex along by telling her his new relationship was only temporary, and she got angry and contacted the new girlfriend, to tell her that the man has been seeing them both at the same time.

do you know of any instances where someone you know who was “the other man” or “the other woman” contacted the spouse or S.O. and told them what their spouse has been up to?