Hilchos Lashon Hara chapter 8, section 8

There is permission to speak lashon hara about baalei machlokes (people who start and keep up quarrels). This is only if he sees that by telling people how great the dishonesty in the quarrel is, they will see that the quarrelers are not right. Then they will not join the quarrel, and so the fight will end. But the following conditions must be met:

1. He must know by himself the facts that show they are quarrelers, and not rely on what he heard from others.

2. He must intend to help, and not speak out of hatred.

3. If he can calm the fight in another way without speaking lashon hara, for example by correcting them, it is forbidden for him to speak lashon hara about them. Unless he is afraid to correct them, because then the quarrelers will see that he is not on their side, and they will spoil his plan to stop the fight.

But this needs great care and thought. He should not decide quickly which side is right in the quarrel. He should think it through well, according to the Torah, to see who is right. And if he is in doubt, it is better to sit and do nothing, and not speak lashon hara, even to stop the quarrel.