Hilchos Lashon Hara chapter 10, section 5

If you see someone speaking lashon hara about another person, that is also a wrong done to that person. So all the rules above about speaking against a wrongdoer apply here too.

But this is only allowed if the person who was spoken about already knows about it. If he does not know, you may not tell anyone. The news will spread, he will hear about it, and that will be rechilus (telling people what others said or did about them), as explained later in Hilchos Rechilus.

And certainly you may not tell the person himself what was said about him, even if you mean well. That is clearly rechilus.

This is true even if the lowest person mocked a very important person, such as your father or your Torah teacher.