NUM428 "You shall not pollute the land wherein you are". This verse forbids us to flatter a wrongdoer (The Gaonim RAM Baal Hatofos and RSH Ben Gabriol). If you know that Reuven dislikes someone, the correct thing to do is to admonish Reuven for his hatred. By speaking loshon hora to Reuven about his enemy in order to find favor in his eyes, you violate this prohibition. A listener to loshon hora can also be guilty of this prohibition. It is very common for people to nod their heads or vocally show approval when someone tells them loshon hora. This flattery is terms chanifas and is a very serious offence (see Shaarey Tshuvah 3:187-199).