It depends. First, define cheating; IRL, online, a hug, lunch/dinner with someone other than their SO?
What if the couple has an open relationship and hasn't necessarily advertised this (remember, we aren't always privy to all agreements)?
Okay, on to your question. What would I do? I'd think about what I've typed above. If I'm unsure of their relationship dynamic, I wouldn't say a word. But, that's just me.