Kural - 820

In anywise maintain not intercourse with those,
Who in the house are friends, in hall are slandering foes

Explanation
Avoid even the least approach to a contraction of friendship with those who would love you in private but ridicule you in public.

Tamil Transliteration
Enaiththum Kurukudhal Ompal Manaikkezheei
Mandril Pazhippaar Thotarpu.