Last evening, a friend and I were at a local diner having dinner and going over next steps on a project. We were seated in a back booth that has a full view of the place. You have to pass this booth to get to the restrooms down a short hall.
During the course of our meal, my friend looks up and gasps. I froze, thinking two masked men just walked in with guns drawn. I turn to see what she is looking at. Nothing out of the ordinary. But then she says, "oh my - there's my cousin's husband and that isn't my cousin with him". I went through the "maybes" - maybe it's a coworker, maybe it's a family member, maybe, maybe, maybe. The maybes stopped when CH (cousin's husband) dunked his finger into a drink, stuck it in the woman's mouth and put it back in his mouth. Then they smiled at each other and began holding hands across the table.

My friend yelped and was about to charge full steam ahead when CH spotted her and walked over to our booth. At that point, I was totally ignored and the conversation went like this:
CH: Hi, what are you doing here?
Friend: (arms crossed and blushed) No, the question is what are you doing over there?
CH: Having dinner with a friend. People do eat, you know.
Friend: Go away CH. You should leave, you and whoever she is.
CH: Whoever she is, is not your business. Okay? (a bit menacingly, but not overly so)
Then he put his finger to his lips like "shhhhhh", walked back to his table and the woman he was with glared at us. They grabbed their coats and left.
Friend was beside herself. She never had any bad inklings about CH, nor had her cousin said anything to her about trouble in their marriage.
She wants to tell her cousin, but decided to wait a few days to calm down. I didn't know what to tell her as I've never been in this situation. I would imagine if you do plan to tell someone a thing like this, it's best to do it as soon as possible - not weeks, months or years down the line. What is the etiquette in this? Tell? Don't tell?
FYI: Friend and her cousin are really like sisters, grew up together and remain close. Cousin lives an hour away from our town.