Hilchos Lashon Hara chapter 10, section 5
If one sees someone speaking lashon hara about someone that is also considered perpetration of harm and all the laws of disparaging the perpetrator mentioned above apply. This only applies if the victim knows he was spoken of. If not, one may not say the matter to anyone as the news will spread, the victim will hear of the matter, and this will count as rechilus as explained later in the laws of rechilus. Certainly, one may not reveal the matter to the victim even if one’s intent is for benefit as it is certainly rechilus. This applies even if the lowliest person mocked an important person who is one’s father or Torah teacher.