It comes down to understanding where loyalties lie. A married man's first and primary loyalty must be to his wife, and to their relationship. You don't have a real marriage without that. And hers must be the same, or the marriage is in serious trouble, even if it doesn't seem so at the time. In this scenario, by remaining silent about it, she has compromised her main loyalty, whether she knows it or not. She has placed the well-being of her husband's friend above her husband's own well-being, under the false guise of "protecting" her husband. She's not protecting her husband, she's protecting his so-called friend, which means that at some level, she responded subconsciously to his advances. That spells trouble in capital letters, because when the truth comes out, the husband will not only feel like a fool, he will feel betrayed, and by the one person in this world that should never do so. That's a clear definition of being blind-sided, and I can tell you this from my own experience, there is nothing that hurts more than being blind-sided by the one you love. Betrayal is a nasty feeling, and it can set into motion a lot of unlovely events. I would venture to say that in many cases, a man's cheating can be traced to a betrayal of trust somewhere in the past on the part of his wife, and it may have been done innocently, with the best of intentions. By not seeing it for what it is, all kinds of problems can result.