I would tell I can't justify a death sentence
hell yea, whats a freind for
I'd tell them. I'm all for minding my own business, but when someone's health is at risk, that's just ridiculous. Even things like herpes that seem innocuous can be dangerous in rare cases.
I would ask them if they are aware and what I had heard and how I heard it.
I'd tell my friend only if I was 100% sure their partner were cheating, STD or not... if it were my friend who were the one cheating, I'd say nothing... just stay away from the couple together.
Probably would depend how I found out. Either way, I'd try to get some photos as evidence. If I'd stumbled on it, I'd show the friend. If I'd found out because I hadn't been minding my own business, I'd probably show the partner.
I'd tell them. Fúck the cheater.
mind your business ... stop being their partner ... and let karma deal with them