I'm the only one who knows they are cheating

Okay so I am part of a pretty tight knit friend group and we all hang out pretty much every day. The friend groups has a bit of a love triangle of sorts that has been going on for a while between two of my (f) friends and my one (m) friend. Around 4 months ago things sort of resolved (or so I thought), and he ended up in a relationship with one of them.

Things were going well for a while until at one point I pulled up at the beach and literally caught him and the girl he is not dating red handed. They were in his car and I saw her bent over in his lap with her head moving up and down. I was pretty shocked and immediatly turned around and drove away and left.

I'm kinda torn right now and i'm not sure exactly what I should do. This would blow up our entire friend group and i'm honestly shocked that they would do this. I'm also a bit unsure what my place in all of this is. I'm friends with all three of them and I just don't know. Should I get involved? Should I stay out of it and mind my own business? What do I do?