[Stress Puppy]

(This is not directed at anyone in specific, I'm just ranting a bit)

Yes, chances are if you fess up to the friend, you will be blamed by one of them. That's human nature.

However, knowing that this behavior is happening and not informing the friend is wrong. It's not a matter of 'it's not my business'. It became your business when the spouse tried to sleep with you. They involved you, and you are obligated to inform the friend that they are involved with an untrustworthy person. Keeping that back is a lie of omission, and furthermore, you're contributing to the deception being perpetrated on your friend.

Also, think of it this way, for those that worry about the other problems that would occur: What happens when the spouse gets an STD? What happens when the spouse gets pregnant/gets someone pregnant? What happens when the spouse finally decides that strange is all s/he wants? By not informing the friend, the impact is worse down the road. And, to add salt to the wound, you then have the knowledge that you knew and didn't tell the friend so that they could make a choice as to what to do about it themselves.

People deserve to be informed about all aspects of their life. Stop trying to protect or baby others. They are fully capable of making decisions about their life.

If the spouse was a car thief, would you tell the friend? If he were a drug addict, would you tell? If he simply talked shit about the friend behind their back, would you tell?