Hilchos Rechilus, chapter 1, section 4

Rechilus is forbidden even if it is completely true, with no lies at all.

If your rechilus ruins a friendship between the person spoken about and the listener, you are called a wicked person, and Hashem finds it disgusting. As it says: "Six things the L-rd hates and seven are the abomination of His soul… And he who causes quarrels between brothers" (Mishlei 6:16, 19). The sages say (Vayikra Rabba, Metzora 16) that this seventh one is the worst of all.

But rechilus is forbidden even if the two people already hated each other.