I'm going to argue with my best mate's girlfriend
Why do you want to argue with her?
Because he's my best mate and the girl doesn't want to listen to me telling other people I don't think they make a good couple.
You should do your best to not argue with your best friends partner out of respect. Since this is your best friend, it would mean a lot to him for you to get along with his partner.
I'll change my best friend if the bitch doesn't listen to me.
If you're that willing to change who your best friend is, maybe you two aren't that close after all. And, if he really is your best friend, you should try to get to know his girlfriend instead of calling her a bitch. If you still don't like her, then you should have a private conversation with your friend about what it is you don't approve of with his girlfriend.