No I disagree. I could see if she had an std or pregnant, that would make it necessary, but since she full well knew what she was doing...she really had no right IMO. It was vindictive and she took pleasure in doing it...that says a lot for her character. She should have kept her legs closed and given him a lecture on how much of a bad idea it was and shut him down.
I have been witness to infidelity (close friends, co-workers and family) I still believe in keeping my mouth shut and minding my own business. It's up to them to work it out.