I'm also torn about this one. If I discovered that my spouse was cheating, and that a couple of my closest friends had known about it all along, I would certainly feel hurt and betrayed by those friends. And Prudie suggests that the wronged wife may already know about it, but nothing in the letter suggests so. The wife has questioned her husband's relationship with this 'other woman', sure, but it may only be jealousy or occasional suspicion, rather than certainty. Her husband is having an 'exciting' time at her expense, and she has a right to know.