What does it mean when a girl dreams about her boyfriend, that he is with another girl (the girl is my friend but he never met her)?

Meaning?: Someone with more info from you might make a stronger interpretation, but i'd say that the relationship is important to you and you're a bit concerned about it. You may be looking for a pov outside yourself to be more objective or to safely try out feelings of jealousy. Who know? - maybe try some counseling just to explore your feelings in real life. Good luck.

You know best: The dreamer is usually the best interpreter of the dream -- so you know better than any of us. However, someone who is jealous of her friend or fearful her friend is more attractive, might dream such a dream. Many other possibilities also. If these dreams trouble you, you could talk about them with a psychologist and learn much more about yourself. Best wishes.