I'm not going to stand up for my girlfriend when my best friend says what he thinks about her.
Would she stand up for you?
Yes, she would, but I don't want to do that.
That seems wrong. You should stand up for your girlfriend and not let your friend say whatever he wants.
Not everyone has to do everything for you. I believe everyone should do things on their own.
It might hurt your partner if you don't stand up for her. It doesn't mean you or your friends have to be dishonest, but how do you think it would make her feel to not have you stick up for her?