This isn't about information sharing, dude. You have to think outside the realm of IT, and realize that leverage is a very real thing, wielded at times to accomplish or attain a result.

It's kind of like getting a loan approved because of your credit score. The bank sees what kind of leverage you have against your debts and therefore sees fit to engage in a transaction with you. (DISCLAIMER: Probably not the best example)

The cheating wife would have no leverage over you, and you will have all leverage over her, without violating any ethical code of conduct. If she comes back to threaten you, you are covered because you didn't stray outside the boundaries of privacy: the only person you communicated with was the person *who originally came to you.* From there, you don't have to do a thing: the mere ability to do it if you so chose exists in her mind, and she likely doesn't know about the strictness of your internal IT policy. "He knows" is sufficient to causing her to realize she can't conceal the truth forever.