Cheating on the Instant Messenger

Ok, so I need your best advice. I have a friend, and she has a problem. And I can't decide what to do. I'll do my best to preserve her confidences.

See, say there was a certain person, who was your friend. And say that this certain person was married to a certain other person, who is also your friend.

And say that certain person told you something, in confidence. See, that certain person has a weakness, an addiction really, that isn't really good for that certain person. And you know that certain other person would not want that certain person engaging in conduct involving that addiction. But that certain person was cheating on that certain other person.

See that certain person told you that they had engaged in that addiction just then and felt that she had carte blanche to cheat because the certain other person did not know.

I mean, that certain person has goals and potential, but that certain person just keeps falling to this addiction, and I feel dirty not telling that certain other person that his this certian person is cheating and telling me about it.

I mean, I should tell that certain other person, right? Wouldn't you?