Shemiras Halashon Halachos
Hilchos Rechilus, chapter 1, section 4 Rechilus is forbidden even if absolutely true with no falsehood. Even one's rechilus destroys a friendship between its subject and the person spoken to one is called a wicked person and is held abominable by G-d as it says (Mishlei 6:16, 19), Six things the L-rd hates and seven are the abomination of His soul… And he who causes quarrels between brothers. The sages (Vayikra Raba, Metzora parsha 16:61) that this seventh is the worst of them all. However, rechilus applies even if the two people hated each other already.
Hilchos Rechilus, chapter 1, section 5 It makes no difference if one spoke willingly or if someone understood some of the situation himself and persuaded one to say the rest of what was spoken of him. Even if one's father or rabbi try to persuade him, and even if only the dust of was rechilus is involved, one may not listen to them.