my friend is obsessed with her boyfriend

K so I'm pretty pissed off. My friend is OBSESSED with her boyfriend and insists on taking him everywhere with her. One time she brought him round to my house without telling me when I only said she alone could come.
This guy isn't even nice, he's EXTREMELY controlling and emotionally manipulative and overall just a huge dickhead. He's tried to get off with me and many other girls numerous times, which she knows about and has forgiven him every single time. 
 We're 16 years old and they've been dating for just over 2 years and I get they're in love or whatever but I honestly don't care anymore because I'm realising how absolutely ridiculous this is and I just can't stand it anymore. 
How can I tell her that she's being stupid and she shouldn't be so attached to her boyfriend?