<human>: Can you condense the text into a few key points?

They've been dating for well over a year now. I live with BOTH of them plus one more roommate.

To put it shortly, I've discovered my friend is bi-curious and is secretly trying to hook up with other men on Craigslist behind our backs. His girlfriend lives with us and she does not know this.

As far as I know, my other roommate is not aware, and my bi-curious roomate does NOT know that I know he is experimenting and being unfaithful.

WHAT DO I DO REDDIT? I don't want to have an uncomfortable conversation and directly cause a problem in their relationship.

BUT, I don't want to do nothing and then watch my friend get away with his infidelity.
IMPORTANT: I have no issue with his sexual orientation. I DO have a problem with his infidelity. I can give all the finer details if you redditors ask specfic questions. Just didn't want a huge wall of text.
<bot>: Roommate is secretly bisexual and has been cheating on his GF with other men on one or more occasions. He doesn't know I have this information, and his GF is unaware. What do I do??