Bad influence on my son

My 16 year old son has a friend I believe is a bad influence on him. He is very protective over this girl, and won’t hear a word said against her. He has known her for a long time and as he gets older they have gotten closer.

Whenever he is with this girl my mind goes to all the ways she is bad for him, and how much better things would be if they weren’t friends.

I can’t influence him to not be friends with her, or even for him to see the things I don’t like about her. She has habits that I worry will become his habits, too. Please help.