You know what is crazy? It’s crazy when people become so comfortable with talking about others that they find it acceptable to relay anything. And yet they find it questionable when it comes to someone discussing others with evidence to warn others away from that person. Subhan Allaah! Anyways, Alhamdulilaah here is the evidence from Islamweb.net
It is permissible for a person to publish a conversation or correspondence between him and another person as long as there is no Sharee‘ah impediment such as being asked not to disclose it. In such a case, it is impermissible to publish it except with his permission, as the Prophet , said, “There is no faith for a person who is not trustworthy, nor is there a religion for a person who breaks his covenants.” [Ahmad with a good chain of narrators]
This ruling applies only to the cases where there is no Sharee‘ah-approved benefit upon publishing such a conversation such as exposing the false beliefs or immorality of someone to warn Muslims against his evils.
In such a case, publishing these conversations may be obligatory as advice for the Muslims.
The Prophet , said, “Religion is advice”. They said, “For whom?” He replied, “For Allaah, His Book, His Messenger and for the leaders of the Muslims and their masses.” [Muslim]