In the past, my friends have merely given someone their number and then nothing happens from there. In the few cases where a line of betrayal has been crossed like, let's say a kiss, I didn't tell their significant other because I did not have a relationship with them. (Literally, we're not friends on Facebook, Instagram, Snapchat, Twitter, or IRL.) I especially don't feel obligated when I know that said person will not be in the picture long-term. It sucks, but that's how it is for me.
It's really tricky if your friends date each other and you're all in the same friend group though. This one time in college, a former friend of mine was dating this guy she met at a frat party—she was starting to develop strong feelings for him and really saw a relationship blossoming before her eyes. Over the weekend, I found out that her roommate had hooked up with the same guy during some sorority field trip thing. She didn't tell my friend, but I found out because the guy was in the same fraternity as one of my friends. Now mind you, this guy and my friend were not in an official committed relationship, but I didn't want my friend rushing into something serious with a guy that she clearly didn't know as well as she thought she did.
Personally, I would expect the people I surround myself with to come forward and tell me if something shady was going down with someone I'm involved with so I try to do them the same courtesy. I went to my friend and told her about what happened because I cared about her and didn't want her to get hurt. I mainly expected her to terminate her relationship with the guy, which she did, but unfortunately she ended up having a big falling out with her roommate too. It was so bad that she had to move out (which I did not see coming) and I felt terrible because I basically caused the end of a friendship.
Shortly after this all happened, I made amends with the roommate. She wasn't mad at me for what happened, but wished that I had come to her first when I found out about it. After that experience, I started to re-think how to approach these types of situations. If the relationship is serious, I think someone has to step in for the sake of that human being. Nobody deserves to be treated like crap. If being directly involved isn't how you want to go about it, you can try to find another way. If someone was cheating on my friend and I had evidence, I would show and tell them without hesitation. If one of my friends was in a relationship with another one of our friends and cheated on them in front of me, I would have to say something but would probably try to approach the cheater first. Obviously, don't leak their business on the Internet though. That's very immature.