LEV1131 If an uncommitted Jew is totally unaware that a particular action is a sin in Judaism, then the transgression performed is done out of ignorance and it is not judged as harshly as for that of a person who is aware of the sin and still transgresses. If a Jew approaches an uncommitted Jew and informs him or her that a particular action is a sin, and this person chooses to continue the action, then the person has been changed from an accidental, unaware sinner into an aware sinner. The act of telling the person who continues actually makes matters worse. That is why the obligation to inform the person exists only if there is a possibility of change. If change is not all realistic, then it is indeed not permitted to inform a transgressor that here she is committing a sin (Shulchan Aruch, Orach Chaim 608:2 and Ramah). The same concept can be demonstrated from the talmudic passage (Sanhedrin 27b) that says that each Jew is bound to every other Jew. This is based on [this] Torah verse that seems to say that one Jew is punished for the sins of another Jew. The Talmud then asks: How can this be--is it fair to punish one Jew who did not commit the sin? The answer is that if a Jew could have prevented this sin from taking place, then indeed he or she is punished (although not to the same degree as the actual transgressor). If there was no possibility of preventing the sin, the other Jew is not punished. Only if a Jew can prevent another Jew from sinning is there an obligation to step in. If not, there is no obligation.