Kural - 821

Anvil where thou shalt smitten be, when men occasion find,
Is friendship's form without consenting mind

Explanation
The friendship of those who behave like friends without inward affection is a weapon that may be thrown when a favourable opportunity presents itself.

Tamil Transliteration
Seeritam Kaanin Eridharkup Pattatai
Neraa Nirandhavar Natpu.