Kural - 187

With friendly art who know not pleasant words to say,
Speak words that sever hearts, and drive choice friends away

Explanation
Those who know not to live in friendship with amusing conversation will by back-biting estrange even their relatives.

Tamil Transliteration
Pakachchollik Kelirp Pirippar Nakachcholli
Natpaatal Thetraa Thavar.