Principle 8 8from Chafetz Chaim

וְדַע, דְּמַה שֶּׁיֵּשׁ אוֹמְרִים, {טז} דְּמֻתָּר לְסַפֵּר לָשׁוֹן הָרָע עַל בַּעֲלֵי הַמַּחֲלֹקֶת, (יז) דַּוְקָּא אִם הוּא רוֹאֶה, שֶׁעַל יְדֵי שֶׁיְּגַלֶּה לִפְנֵי אֲנָשִׁים אֶת גֹּדֶל תַּרְמִיתָם בְּזֶה הָעִנְיָן, וְיִרְאוּ שֶׁאֵין הַדִּין עִמָּם, מִתּוֹךְ זֶה תֻּשְׁבַּת הַמַּחֲלֹקֶת, אֲבָל בְּלָאו הָכִי {בלי זה} אֵין שׁוּם חִלּוּק בַּדָּבָר. גַּם צָרִיךְ לְאֵלּוּ הַפְּרָטִים שֶׁאֲבָאֵר:

And know, that others' saying that it is permitted to speak lashon hara against "men of contention" applies only if by revealing to men their [the contenders'] great deception in this matter and they see that the din is not with them, the contention will cease. But, failing this, there is no difference in the matter. The following details are also needed [for a heter]:

א שֶׁהַדְּבָרִים, שֶׁעַל יָדָן הֻסְכַּם בְּדַעְתּוֹ שֶׁהֵם בַּעֲלֵי מַחֲלֹקֶת, יֵדַע מֵעַצְּמוֹ, וְלֹא יִסְמֹךְ בָּזֶה עַל שְׁמִיעָה מֵאֲחֵרִים, אִם לֹא (יח) שֶׁנִּתְבָּרֵר לוֹ שֶׁהוּא אֱמֶת.

a) that the things which convince him that they are "men of contention" be known by himself and not by reliance upon what he hears from others, unless it is clear to him that what they say is true.

ב צָרִיךְ (יט) שֶׁיְּכַוֵּן לַתּוֹעֶלֶת הַנַּ"ל, וְלֹא מִצַּד שִׂנְאָה.

b) He must intend the aforementioned benefit and not be motivated by hatred.

ג אִם בְּאֶפְשָׁרוּתוֹ לְהַשְׁקִּיט הַמְּרִיבָה (כ) בְּאֹפֶן אַחֵר, שֶׁלֹּא יִצְטָרֵךְ לְדַבֵּר עֲלֵיהֶם, כְּגוֹן, עַל יְדֵי (כא) הוֹכָחָה וְכַיּוֹצֵּא בָּזֶה, אָסוּר לוֹ לְסַפֵּר לָשׁוֹן הָרָע עֲלֵיהֶם, אִם לֹא שֶׁהוּא יָרֵא לְהוֹכִיחָם, פֶּן כְּשֶׁיֵּדְעוּ בַּעֲלֵי הַמַּחֲלֹקֶּת מִזֶּה, שֶׁאֵין דַּעְתּוֹ עִמָּהֶם, יָפֵרוּ עֲצָתוֹ, וְשׁוּב לֹא יִהְיֶה לוֹ עֵצָּה, אֵיךְ לְתַקֵּן אֶת הַדָּבָר, אַךְ בְּעִנְיָן כָּזֶה צָּרִיךְ שִׁקּוּל הַדַּעַת גָּדוֹל, וְלֹא יְמַהֵר לְהַחְלִיט הַדָּבָר בְּדַעְתּוֹ לְהַטִּיל שֵׁם בַּעֲלֵי הַמַּחֲלֹקֶת עַל צַד אֶחָד, רַק שֶׁיִּתְבּוֹנֵן הֵיטֵב עַל פִּי הַתּוֹרָה, מִי הֵם בַּעֲלֵי הַמַּחֲלֹקֶת. וְאִם אֵינוֹ יָכוֹל לְבָרֵר בְּדַעְתּוֹ הַדִּין עִם מִי, שֵׁב וְאַל תַּעֲשֶׂה - עָדִיף.

c) If it is possible for him to silence the quarrel in a different way, without having to speak against them, such as reproof and the like, he is forbidden to speak lashon hara against them — unless he is afraid to reprove them, lest, when "the men of contention" see that he is not in agreement with them, they will annul his counsel until he is left with no further recourse to settle the matter. But in such an instance, he must exercise great discretion and not hastily decide to call one party [to the dispute] "men of contention." But he must judge carefully, according to the Torah, who are the "men of contention." And if he cannot decide who is right, "Sit and do nothing" is the preferred course.