so i have this tiny wee problem

my best friends boyfriend keep hitting on me while she is not around. hes even too smiley when she is around and i can tell theres been instances where she got jealous of that. now they have been dating for 5 years and are currently engaged. recently he started grabbin my ass and tits while she is not around. if i threaten him he says that she wont believe me anywas .and he is completely right. my friend is very insecure and completely in love with this fool . she already chooses to put up with his rude comments and very controlling behavior. i have a feeling that if i tell her the truth she will dump me as her friend and not him. he completely controls her and there is no way she will break up with him . hes such an asshole i just wanna kick him in the nuts !!

and why oh why is my friend sooo pathetic

what should i do ?