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.