Moral Dilemma - In the Bar

Problem:
The problem that I chose to discuss the opposite of is "In the Bar". I see my friends spouse enter the bar with another person, do I tell my friend? Do I tell my friend what I witnessed between the two?

Claim:
I originally replied to this, yes, I would tell my friend immediately what I witnessed. For the purpose of the exercise, I will claim the opposite.
So, no I would not tell my friend about what I witnessed, and I would mind my own business/keep to myself.

Reason:
The reason that I would not tell my friend about what I witnessed in the bar is because I do not know the whole story. I do not know the back story of this situation. It is possible that my friend and their partner are swingers. It is also possible that the couple had been going through a rough spot, and just now are decided to recommit to each other and may be ending any affairs at that very moment. In that case, telling what I saw would be something that tore them apart and could make matters worse. As opposed, to letting them run their own course, and work it out on their own. 

Principle:
Bottom line is, people should mind their own business unless it is a life or death situation. Getting involved in situations that do not directly concern you can lead you down a dangerous and slippery slope. Not having 100% of the information makes the situation circumstantial and not completely valid. The old saying isn't always true, perception is not always reality.