Sefer Chofetz Chaim chapter 8 halacha 8

Based on a Yerushalmi in Peah 1:1 which the Smag brings down in his mitzvos and the Hghos Maimoni mentions but the Rif, Rosh, and Rambam leave out so the Chofetz Chaim just says this is according to one opinion, that it is permissible to speak lashon hara about an argument ensuing but only if the purpose of speaking lashon hara is to diffuse the argument by applying pressure to one side or both sides to break up the fight.

The source of this is by King David when he appointed Shlomo to take over the kingship and Adoniyahu started a fight vying for the kingship. Someone told King David about it and he immediately diffused the argument. Also, Moshe sent a messenger to Dasan and Aviram who started a rebellion and the messenger reported back to Moshe in order to squelch the argument. But it might be there that because there was already a rebellion ensuing then obviously the messenger could tell Moshe. But we see from here that the messenger of the court can report back even lashon hara to the court in order to enforce order.

But there are 3 conditions before speaking lashon hara:

1. You have to have seen the argument ensuing first hand or if you just heard of it second hand you have to have clarified the truth to be sure there is really and argument ensuing.

2. Your purpose for speaking out, and maybe even taking sides must be only good intentions, to break up the fight, but you can’t say anything if it’s said out of hatred.

3. If there is a way to break up the fight without speaking lashon hara, for example approaching them and rebuking them that is better unless it will stoke the coals and worsen the fight.

 However, one has to be very very very careful to thoroughly and clearly go through the matter to make sure you are clear what is happening before taking sides and speaking out in order to order to pressure the argument to stop because even if one step was overlooked it could change everything and backfire. 

Especially if the one talking out is a rabbi of clout, it can cause a big chillul Hashem, besides the lashon hara, and making the matter worse so in that case it’s better to just be passive, stay quiet and don’t act.