The Chafetz Chayim, in his famous work for which he is named (“Chafetz Chayim”) dedicated to the laws of lashon ha-ra (disparaging speech about other people), presents the guidelines for when lashon ha-ra is allowed for the sake of preventing harm (Issurei Rekhilut, 9). He begins by establishing that if a person learns that his fellow is considering entering into an arrangement with another individual which he knows will prove detrimental, he is obligated to share the relevant information with his fellow. The Chafetz Chayim (in Be’er Mayim Chayim, 1) gives the example of somebody considering hiring a certain domestic helper, and another person knows for a fact that this prospective employee has a history of committing theft. In such a case, the Chafetz Chayim writes, the person with this information is not only permitted to share it with the person who would otherwise be putting himself at risk, but is required to do so.
However, the Chafetz Chayim emphasizes that five conditions must be met for the sharing of this negative information to be permissible:
1) The individual must not instantly decide to share the information, and must instead take some time to think carefully and ensure that this is warranted. The Chafetz Chayim refers us here to an earlier passage in his work (Hilkhot Lashon Ha-ra, 10, note 6) where he explains that sharing negative information about another person must be presumed forbidden until it is ascertained that it is permissible under the circumstances. Therefore, one may not divulge negative information even for an important reason before carefully determining that this is permissible and necessary.
2) He must not embellish in presenting the relevant negative information. He may share only that which is necessary to save his fellow from harm.
3) His intention must be solely to protect his fellow from harm, and not to smear the other party. The Chafetz Chayim clarifies (in Be’er Mayim Chayim, 3) that this does not mean that one whose motives are less than pure should refrain from sharing the information. After all, one who has information that could save his fellow from harm is halakhically required to share the information. Rather, it means that he must force himself to overcome whatever feelings of hostility he might have so he speaks purely for the right reasons.
4) There must be no other possible way of protecting the vulnerable party from harm. Earlier (Hilkhot Lashon Ha-ra, 10, note 11), the Chafetz Chayim drew proof to this concept from the Gemara’s discussion in Masekhet Sanhedrin (11a) regarding the story told in Sefer Yehoshua (7) of Akhan, who stole some of the riches seized from the city of Yericho. God had commanded destroying all the property of Yericho, and He punished Benei Yisrael’s for Akhan’s violation by causing them to lose their next battle. God then informed Yehoshua that the defeat occurred because somebody stole forbidden property, and the Gemara relates that Yehoshua wanted God to tell him who it was so he could be reprimanded and the property returned. God replied, “Am I a talebearer?!” Instead of directly divulging the information, God instead had Yehoshua cast lots to find the culprit. The Chafetz Chayim notes that although Akhan committed a serious offense that had grave repercussions for the entire nation, and the Gemara elsewhere (Sanhedrin 44a) relates that Akhan also committed numerous other grievous transgressions, God refused to identify him by name, and had Yehoshua find out through other means who stole spoils of Yericho. This thus proves that divulging negative information about somebody for a vitally important purpose is forbidden if the purpose can be achieved through some other means.
5) Finally, sharing the information is allowed in such circumstances only if it would not result in grave harm to the person about whom it is spoken. If one knows that his fellow is considering a work arrangement with a third person that would be detrimental to him, he must inform his fellow of the danger – but only if the consequences will be limited to the prevention of the given arrangement. But if the fellow who hears the information will then spread it to others, who will rush to unjustly condemn and ostracize the individual in question, then sharing the information is not justified.