Most Helpful Girl
In life you will always encounter people who will not always value your opinion because you cannot please everyone in the world. You have to believe in yourself and if you like this girl you should not care about what her friend thinks of you.
Over time, your girlfriend’s friend will come to accept you especially if she values her friend but don’t let her push you around. It is possible that your girlfriend’s friend maybe jealous of you in someway.
Just don’t bring it up around your girlfriend's friend because it will show weakness and her friend will use it against you. If your girlfriends friend still bothers you discuss it with your girlfriend and tell her how it is affecting your relationship with you and ask her to discuss it with her friend.