My friend speaks badly of my boyfriend all the time, what can I say to her?

My boyfriend and I have been dating for a little over two years, and he's really a great guy. I have this friend though, whom I do enjoy her company, but she thinks it's okay to talk bad about him in front of me. They aren't good friends by any means, only because he doesn't really like to put up with her as she can be quite dramatic. She'll tell me he is immature, rude, I can do better, etc. All I really say is "No he's not," but I'm not really sure what I can say. How do I tell my friend to shut the hell up without coming off as mean, because she does get offended super easy (outspoken and offended easily totally clash, I know!) Thanks for any advice!