Yeah true on the last point. I also get your motives, but it backfires.
Telling a person that the one they may love most is cheating if they believe it. They might not believe the person telling them. Hell I've known situations people upon getting that information excuse themselves and go get a gun and thank their cheating spouse the person that told them and themselves. Some times just themselves. Granted those are extreme cases but most are likely not to want to believe or at least the person is telling them for the reasons they say.
I guess one could send them proof anonymously, but again that could go very badly.