Leshon hara', expressing to someone else negative comments about another person, is considered a very serious offense in Judaism. So much so that the rabbis compared leshon hara' with murder or "character killing".
Leshon hara' is forbidden even when the negative information we say about a person is true.
There are, however, some exceptions. Cases in which disclosing truthful negative information is permitted, or even mandatory.
The simple rule is that you could/should disclose negative information about A, if you do so in order to save B from harm.
Illustration: Suppose Mr. B asks you about Mr. A's integrity. Mr. B is about to engage in business with Mr. A, and you know beyond any doubt, either by personal experience or by direct knowledge (not by rumors!) that Mr. A conducts his business inappropriately. In this case, you should tell Mr. B the truth about Mr. A in order to protect Mr. B from harm. Some rabbis would say that even if Mr. B does not ask you for a reference about Mr. A, if you found out that Mr. B is about to engage in business with Mr. A, you must warn Mr. B.
Now, althought in this case it is allowed for you to speak negatively about Mr. A, when you tell Mr. B the negative information about Mr. A, you must make sure that:
1. You don't exaggerate the facts or overstate Mr. A's misconduct.
2. You really have a pure intention in mind. You are saying these words just to prevent Mr. B from harm. You are not disclosing this information out of feelings of revenge or resentment towards
Mr. A, or because of a personal interest in Mr. B's business.
Everyone should take this license with utmost seriousness. Keeping in mind that our Sages considered that leshon hara' leads to very destructive consequences, even "character killing".
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