Quotation for Arakhin 31:47

And also, if he sees one speaking lashon hara against his friend, this, too [i.e., speaking lashon hara], is one of the transgressions between "man and his neighbor." Therefore, if all the above details have been satisfied, it is permitted to publicize the great wrong of the speaker. And all this does not avail [to permit it] unless the thing has already come to the ears of the one spoken of. But if not, he may not relate it even to others, for "your friend has a friend, etc.," and it is bound to be revealed to him, too, and is in the category of rechiluth. How much more so is it forbidden to reveal it to the man spoken of himself, even if his motivation be zeal for the truth. For this is absolute rechiluth, even if one of the low-life mocks one of the most eminent men in Israel, even if he be his father or his teacher.