It may be that what your girlfriend expects of a partner is more than what she expects of a casual friend. I'll bet she didn't particularly enjoy the friend's remark. But she doesn't want to make an issue of it. With you she is more comfortable in being very open about what bothers her. I think you might consider taking the double standard as a tribute to your relationship being far more special than any other relationship in her life. She wants to keep it that way.