Not as pissed

Im not as pissed today (or atleast trying not to be). It's not them being together that bothers me, it's that they lied about it for quite a long time. This has been going on since April! AND plus I KNOW for a fact he's cheated on her. So how in the world do I handle that? I mean it would look like im lying since I was pissed off and not okay with it. But she's also a friend and I dont want to see him hurt her. A few days after they started dating BD, A bunch of my guy cousins, kiddo and I all went camping at a campground. We had a motorhome and 3 or 4 tents. Anyways one of my cousins and BD picked up some girls they met a few hours earlier and hooked up with them. It's not hard to hear when someones being loud in a tent. I know no protection was used either. So I mean it's really almost a safety issue to tell her because he's nasty. I've also heard from him about hooking up with random chicks here and there. So do I rat him out? Talk to him about it? Never say a word? I don't know what to do.
Greg has been super supportive with it. It's nice to be able to not have any secrets with someone. He just listens and doesn't judge.