What if you saw your friend's spouse being intimate with another person? Would you tell?
Randy Cohen, who writes "The Ethicist" column in The New York Times magazine, says this is the question most often asked of him.
He does not have a simple answer for this dilemma.
In his book, "The Good, The Bad and the Difference," he says married couples have a range of attitudes towards infidelity: "Some couples demand full disclosure and absolute fidelity; others allow a wider range of behavior."
Ideally, Cohen said, the wronged spouse should be able to say ahead of time whether he or she wants to hear of infidelity. "A sort of living will of the heart," he said. "Check one: I want to know/ I want to remain in the dark."
But that's never practical, he said, so he advises, "If she doesn't want to know, respect that desire. If she does want to know, you owe it to your friend to tell her. But if you don't know her well enough to be confident of her desire, then you don't know her well enough to intrude, so keep your suspicions to yourself."