I was just reading an columnist's article in the Toronto Star where she published readers reponse to the above questions, which were quite diverse.
Now, the answer to this question may seem like a no-brainer (yes, you would tell the person if you knew that their boy/girlfriend was cheating on them) but this can be a complicated dilemma depending on the factors and situations involved. The messenger may have noble intentions but may get a kick in the teeth (figuratively or literally, depending on how pissed off the person is to the messenger) for their efforts. What you think should happen and what actually happens are two entirely different things.
Have you guys ever been in a situation where you knew someone (friend, relative, etc) was cheating on their partner? If so, what did you do?
If you were being cheated on by your partner, would you want someone to tell you? (Be honest.)