I've dealt with something similar where I walked into an ETL and TM going at it with the TM being the cheater.
Personally, I would've had no issues talking to the TM's SO, not just as an issue for morality but because of the risk of possible STD's and the SO being unknowingly at-risk.
But, with no way to contact them, I instead had a long chat with the TM. Did it end things? Probably not, but they knocked off the off-stage canoodling.
As for the snitch-line, I disagree, but that's just me.
As for the issue at hand, you're making a conclusion that, though likely, you have no evidence of.
Taking the responsibility of exposing someone's marital infidelity is a a huge risk. You'll lose the respect and friendship of the people involved and the childish "snitches/stitches" posse that may exist at work. Taking such a risk when you have no proof, and bringing the attention of it to someone other than the SO when it's not impacting the business? Comes off more as a pot-shot than it does being moral.