Muh mein ram ram bagal mein churi
Speak praise on the face and stab him from behind.
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Someone who is two-faced is not sincere, saying unpleasant things about you to other people while seeming to be pleasant when they are with you.
which I think is the most suitable in OP's context.
The closest I can think of is a proverb for a subset of such behaviour where the smiling and stabbing actually take place at the same time, but the hypocracy at least is intact:
damning with faint praise. From Collins Dictionary:
If someone damns something with faint praise, they say something about it which sounds quite nice but is not enthusiastic, and shows that they do not have a high opinion of it.
Dominique damned it with faint praise :'It tastes quite good when you've lived in the UK for 22 years'.
Wikipedia gives the following origin for the phrase:
The explicit phrasing of the modern English idiomic expression was first published by Alexander Pope in his 1734 poem, "Epistle to Dr Arbuthnot" in Prologue to the Satires.
Damn with faint praise, assent with civil leer, And without sneering, teach the rest to sneer; Willing to wound, and yet afraid to strike, Just hint a fault, and hesitate dislike. — "Epistle to Dr Arbuthnot" by Alexander Pope (1688–1744)
Someone like this can be described as a snake in the grass defined as
A treacherous or untrustworthy person
According to the Free Dictionary link above the idiom goes back to the Roman poet Virgil in 32 BC.
The only difference between this and the Hindi idiom is that a snake in the grass can be merely neutral towards rather apparently supportive of the person whose interests are likely to be damaged