The commandment, you shall reprove your fellow (Vayikra 19:17), requires a Jew to inform a sinner that his behavior is improper, and attempt to convince him to mend his ways. In voicing his disapproval, one must be prepared to endure embarrassment and insult and should continue to protest. Only in a case where one knows that his reproof would cause the sinner to react by committing more serious offenses is he to refrain from speaking up.
(Reproof must be administered with respect and understanding, and should be done in private whenever possible – see Rashi to Vayikra 19:17).
Generally speaking, one is required to interrupt and reprove the speaker of loshon hora, and do his best to ensure that he put a halt to his sinful speech. As mentioned, this does not apply when the speaker would likely react by expressing yet greater negativity towards the person he was speaking about, as a way of defending his sinful talk.
Another situation where reproof would be out of place is when:
The speaker does not realize that the information he is conveying is loshon hora, or is totally unaware that loshon hora is prohibited by the Torah; and
it is obvious that the speaker will continue speaking loshon hora even after being told that this is forbidden.
In such a case, reproof would transform the speaker from an inadvertent sinner into one who sins intentionally. Hence, it is better not to reprove him.
