Hilchos Rechilus, chapter 9, section 5

Another important rule is that the leniency of paragraph 1 only applies if the partnership was
not finalized but only agreed on. If it was finalized (according to the various ways this is done)
it depends. If one knows that Shimon will not harm Reuven but only be careful to save himself
from harm, and the conditions of paragraph 2 are satisfied, one may tell him about Reuven and
should do so. But if one knows that Shimon will immediately believe the story (either because
he always believes evil tales about people, or because there are signs the story is true, or because

he relies greatly on the speaker of the rechilus) and take the law into his own hands, illegally
cancelling the partnership or doing other harm, one may not say the rechilus. This applies even
if the harm is no worse than if one said the rechilus together with another witness in front of a
beis din, because as a single person the beis din would not have believed him and caused Reuven
a monetary loss. Accordingly, if two people say the rechilus it seems that such a case would be
permitted as they are causing Reuven no more harm than the law requires. (This does not apply
if they know Shimon will do more harm than the law requires). Also, they must fulfill all the
conditions of paragraph 2.