I love the easy questions!
Don't confuse your own needs and frustrations with those of your girlfriend.
- If she's happy to give rides/money/etc to her friend, respect that.
- If you are feeling neglected/hurt/attacked by the friend's actions or behaviors, express that directly and with clear examples of how you were physically/emotionally injured.
You goal isn't to force or even convince your girlfriend to take your desired action(s), nor is it to lay down some form of ultimatum. Simply be open to sharing your feelings, and your desires around your shared relationship, without stepping on your girlfriend's other connections or her own feelings.
If she cannot listen to your point of view, and is unwilling to work with either or both of you on an amicable resolution, then the real question is this: why stay in a relationship with someone who doesn't respect your feelings, and isn't interested in working on improving your relationship?
And while it should go without saying, be open to compromise. It's not about getting everything you want, but rather about being confident that you are free to share these types of issues/feelings with each other without fear of reproach.
How do I get my girlfriend to stop talking to a girl who is not really her friend?