First I don't think your example really fits, because your example tends to represent the assumption that the report was "grossly inaccurate". While in fact, Joh's account does not specify whether the staged assault included a rape threat or a burglary threat (to that, Knox's first explanation did not explain that either), but offered instead precise details about place (her apartment), context, identity of 'victims' (her roomates), date (April), dynamics ('uses' friends to inscenate a crime, producing terror or distress), and a further detail (ski masks). The prank depicted is roughly identical to Knox's admission, most these pieces of information just match with it.
So I won't speak about grossly inaccurate report. But as for your question involving the wife, I will say the question is set incorrectly: if someone sleeps with my wife, this individual actually has no absolute 'ethical obligation' to reveal anything of any sort. He - as well as wife - are also free to lie.
However, if the person (and my wife) say "no, we didn't do anything", but then I happen to see a video recording showing the man and my wife kissing each other, at that point the problem what happens is that the man and the woman will lose any credibility. It would be too late at that point to say "no we didn't sleep together". Why should I believe them? And at that point, who cares after all? The lack of transparency is cheating.
Cheating is cheating, it is concealing, pulling curtains on facts. The nature of the action itself may become rather secondary, it's the intent to conceal the truth, whatever the truth is, to disguise it, this is what matters to credibility.
This attitude here qualifies a community, as I said.