Shemiras Halashon Halachos

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.

Hilchos Rechilus, chapter 9, section 6

Even when there are two people, Shimon would sin if he acted upon his own initiative for he may not believe them unless they testify in a beis din and the beis din gives him permission to act as explained earlier in chapter 6 paragraphs 9 and 10. Also, it is rare to find such cases (besides the fact that it is rare that all the conditions will be fulfilled) for it is very unusual for the speakers to know all the relevant laws and know whether Shimon would be acting according to Torah law. Therefore, even two people should not say anything to a person who generally takes the law into his own hands without first obtaining permission from a beis din.