If you knew that a friend's significant other was cheating on him or her, would you tell your friend the truth or keep it to yourself?
If I was at least on somewhat friendly terms with the person in question, then it's just a matter of playing to their conscience; you're doing this, and you expect me to sit around and watch? You can't man up and admit you've done this? You're still doing this? What's wrong with you? I'd guilt them into doing it.
If I don't know the person, or - worse - if that doesn't work, then yes, it's time for an intervention.