Hilchos Lashon Hara chapter 10, section 9

Suppose a person is known to everyone as someone who fears no one. Whatever he says about others, people know he would also say to their face. And he is known to speak only the truth.

Such a person may tell about a wrongdoer's misdeed even in front of one or two people. This is true even if he did not correct the wrongdoer first, because he knew the wrongdoer would not accept it.

People will not suspect him of being a flatterer or a liar. They know he cares about the truth and wants to help the victim.

But he must still keep all the other conditions in this chapter.