Part One, The Prohibition Against Lashon Hara, Principle 7, Seif 5 חלק ראשון: הלכות איסורי לשון הרע, כלל ז, ה׳
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וְכָל זֶה, שֶׁדִּבַּרְנוּ, הוּא בִּסְתָם אִישׁ יִשְׂרָאֵל, אֲבָל אִם (י) נִתְחַזֵּק מִכְּבָר בְּלָאו הָכִי לְאָדָם רָשָׁע, מִפְּנֵי שֶׁנִּתְפַּרְסֵם עָלָיו כַּמָּה פְּעָמִים, שֶׁעָבַר בִּשְׁאָט נֶפֶשׁ עַל אִסוּרִין הַיְדוּעִין לְכָל יִשְׂרָאֵל, שֶׁהוּא אָסוּר כְּנֵאוּף וְכַיּוֹצֵּא בָּזֶה, עַל אִישׁ כָּזֶה מֻתָּר לְקַבֵּל לָשׁוֹן הָרָע.

All this that we have said applies to a Jew in general; but if he were already known from the past to be an evildoer, it having been revealed about him several times that he transgressed flagrantly issurim known to all of Israel to be forbidden, such as fornication and the like — about such a man it is permitted to accept lashon hara.