Chafetz Chaim, Part Two, The Prohibition Against Rechilut, Principle 2
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יֵשׁ אוֹמְרִים, דְּאִם אֶחָד (ד) סִפֵּר (ה) גְּנוּת עַל חֲבֵרוֹ בִּפְנִי שְׁלשָׁה, תּוּ {יותר} אֵין בּוֹ מִשּׁוּם רְכִילוּת, אִם יַחְזְרוּ וְיֹּאמְרוּ לוֹ: פְּלוֹנִי דִּבֵּר עָלֶיךָ כָּךְ וְכָךְ. וְהַטַּעַם הוּא, מִשּׁוּם דְּהוּא דָּבָר הֶעָשׂוּי לְהִתְגַּלּוֹת לְבַסוֹף, דְּחַבְרָךְ חַבְרָא אִית לֵהּ, וְזֶה מְגַלֶּה לָזֶה וְזֶה לָזֶה עַד שֶׁיּוֹדֵעַ, וְלֹא אָסְרָה הַתּוֹרָה בָּזֶה מִשּׁוּם רְכִילוּת, רַק צָּרִיךְ לָזֶה הַפְּרָטִים הַמְבֹאָרִים לְעֵיל בְּהִלְכוֹת לָשׁוֹן הָרָע כְּלָל ב' מִסָעִיף ד' וָאֵילָךְ. אֲבָל אֵין לִסְמֹךְ עַל דֵּעָה זוֹ לְמַעֲשֶׂה, כִּי הַמַּהַרְשָׁל בְּבֵאוּרוֹ על הַסְמָג כָּתַב, שֶׁרַבִּים מֵהָרִאשׁוֹנִים (הַלֹא הֵמָּה הָרַמבַּם והַסּמָג והַתּוספוֹת) חוֹלְקִים עַל דֵּעָה זוֹ וְאוֹסְרִים בְּכָל גַּוְנֵי, אֲפִלּוּ לְסַפֵּר לְאַחֵר (ו) אִם מִתְכַּוֵּן לְגַלּוֹתוֹ, וְכָל שֶׁכֵּן לָזֶה בְּעַצְּמוֹ.
There are some who say that if one spoke demeaningly of his friend before three, [the issur of] rechiluth no longer obtains if they go back and say to him: "Ploni spoke this and this rechiluth against you," the reason being that it is something which is bound to be revealed in the end, for "your friend has a friend," and the Torah did not forbid this by reason of rechiluth. But in this, the qualifications must be observed, which were explained in Hilchoth Lashon Hara, Principle II, section 4 and on. But one is not to rely on this opinion in practice. For the Maharshal in his commentary on the Semag writes that many of the Rishonim dispute this opinion and forbid it, even to tell it to another if he intends to reveal it, and certainly to this one [the one spoken of] himself.