If you did nothing wrong and she has no false proof that you did, either from her friend or from some other source, then I would think things would blow over. When they do, she'll owe you big time for doing this on your birthday.
If her friend is saying that you did something when you didn't, then its a matter of her choosing to believe either you or her. If this were a movie, she would listen to her friend and end your relationship. Then, after a passing of time, her friend would do something to make her believe that she was lying all along, and she will come running back to you. By then, though, you will have hooked up with a supermodel in a emotionless, purely sexual relationship. After thinking it over you will choose your girlfriend and elope together to Guatemala.