If the "cheating" does not pertain to yourself and your partner, leave it be. Whoever is involved can figure their lives out for themselves. If it is your friend's partner and you tell them, your friend will likely resent you anyway....people don't always break up after infidelity...you never know how that will play out or how it could be held against you later for bringing it up in the first place. These things are messy. Boundaries are important. I often tell myself: eyes on your own yoga mat!

If you wind up losing a friendship over someone else's infidelity, then that person was not a good friend in the first place. You are not responsible for monitoring other people's fidelity. Let it go and move on with your life. Peace.