Your eyes are not deceiving you. That IS your best friend’s spouse in the corner of the restaurant playing tonsil hockey with some random chick.
So after you have picked up your jaw off the floor, what do you do? Do you confront him? Sneak away and forget what you saw? Or do you call your friend and tell her what you saw?
The decision is a difficult one because someone’s happiness is at stake. On one hand, your friend is (likely) clueless and has no idea how trifling her partner is but when you think about their union, the children involved and just how heartbroken she’d be, you have to wonder if saying nothing may be less cruel. Also, some people do not appreciate the whistle blower for their role in the demise of the relationship and may lash out at the messenger.
So now you have a major decision: do you do the best thing or the right thing? The best thing is the action that creates or causes the least amount of impact or fall out. Maybe you do not want to be the cause of an empire falling so you consider holding your tongue and pray she finds out on her own.
The right thing to is to be a true friend and spill the tea and let the chips fall where they may. Just ensure that as her friend you are there to help her pick up the pieces when everything crashes around her.
If you decide to come clean be prepared for a myriad of reactions, including anger that may be directed at you. Try not to take it personally. It is not about you, you’re just caught in the crosshairs. Just state the facts plainly and avoid the sordid details and hope for the best possible outcome.