What should i do about my best friend's relationship with her boyfriend?

(first off i would like to add that she has stayed at our house at least 3 or 4 times a week since she was 8, so i know her really well and want her to be happy). My best friend is 19 and she started dating this guy who 20 about 10 months. At first I was excited for her. Anyways as it turns out he has a bad rep that doesn't meet my friends high standards. He is nice to you whenever my friend is around, but as soon as she leaves the room, completely opposite. If anyone of her family n friends tries to make conversation with him, he won't answer you, and doesn't care if he's rude to you in your face. just like a really bad impression. But she is way to in to this relationship or infatuated to see what he is really like..she just says that he is shy and doesnt know us. but he has openly told us he doesnt care if he gets 2 know us or not. N he is only comfortable with his friends and super chatty. For the past 10 months she has been coverin up for the way he is acting, views it as nothing.

You just need to tell her as it is. But i think she has already notice it because I think that she is in denial about. Give her time and just talk to her, cause your her best friend and eventually she will listen to you.

u should get ur friends together and have a little intervention type thing and talk to her about y everyone doesnt like him and if she gets mad dont force her to stay just give her some room and point out wat makes this guy so bad so that she pays attention to the sign of y he isnt a good choice for her i hope it ends well

Stay out of it, and let her know that you'll support her no matter what happens.

you don't want her to blame you for anything (she doesn't want to see the truth right now)