As most of you guessed it right, the idiom in question is “a s | BM | IELTS 9.0

As most of you guessed it right, the idiom in question is “a snake in the grass”:

Definition
A snake in the grass: A person who pretends to be your friend but is secretly dishonest or dangerous. This idiom is used to describe someone who is deceitful and treacherous.

Example Sentence
When Julia found out that her trusted colleague had been spreading rumors about her, she realized there was a snake in the grass in her office.