My friend talks in a demeaning way about his girlfriend and I'm going to tell her.
What are your reasons for telling his girlfriend about this?
I hate bad manners and at the same time I don't like the way he is constantly annoying me
Just because you don't like your friend's manners, it doesn't mean that you should tell on him. Talking to his gf about what he says about her is interference in their relationship and wrong. Don't get involved in other people's private lives.
It's not my fault I've been around when he says these things.
Unless you think his girlfriend is truly being harmed, it's best to stay out of your friend's relationship drama. Not only might the girlfriend get mad at you in a "shoot the messenger" type of way, but your friend might also resent you for doing this. You should probably just tell your friend to be more respectful and hopefully he takes your advice.