Hilchos Lashon Hara chapter 4, section 9
A person that hold bad personality traits, for example, that he is haughty or is quick to fly into rage. Although we can see that he got used to these traits and his heart is not embittered because of them, it is forbidden to denigrate him and talk about aspects of his personality that are shameful. Because truthfully speaking, we often see people, even men who are learned in Torah, who don't consider those bad character traits as being a significant sin, and are not aware to the severity of this type of behavior. Perhaps even the sinner is not aware to the severity of this bad behavior, and if he understood the seriousness of the sin that he is committing, perhaps he would make a strong effort with all of his might not to laps back into bad character traits. If one sees this person routinely expressing one of these bad character traits, one is obligated to reprimand him and explain to him the severity of the sin that he is committing. By doing so, one fulfill the mitzvah of "Reprimanding your fellow Jew" and it is possible that this person will concede to him that he was committing a sin. But at this moment this person thinks his behavior is justified and correct. Therefore it is forbidden to go and talk about him to other people.
Hilchos Lashon Hara chapter 4, section 10
Despite it is forbidden to talk about his shameful behavior, it is correct for this observer to tell his (own) son or his students and warn them not to associate with this person so that they will not learn to emulate this person lifestyle. The essential reason why Lashon Hara is forbidden by the Torah is because the speaker's intent is to humiliate his fellow Jew and rejoice in his disgrace. But if the speaker's intention is to protect his friend so that he will not learn to copy this person lifestyle, then it is obviously permissible to disclose this person actions. However in this kind of circumstance it is a mitzvah for the speaker to explain the reason why the discloser is being made, in order that the listener should understand the reason for the comment and not think that this type of speech is ordinary permissible. Moreover, the listener should not think the speaker is contradicting himself since he had said prior that Lashon Hara is forbidden even if it is truth.
