I've been in this situation a couple times
One was a chick I knew in a social circle who was a major hoe, I didn't tell the guy cause 1. I didn't care and 2. If he didn't know his girl was/is a slore than oh well
2nd was my best friend I saw a girl he was dating I told him right away lo and behold they had broken up the week before. Come to find out this girl at a bestiality fetish....