Haha, well thanks for volunteering me, BB.
Well, it is a commonly held belief that dreams manifest concerns or desires in your real life, that are kind of strewn together in the same pot, as your brain's subconscious takes over during the night. It may not be what you want to hear, but somewhere inside, you probably do think her friend is cute, or some similar end.
That being said, it's only a dream... And as long as you never have these thoughts in the day when you guys are together, and you care for your girlfriend, then that is where they will stay... In your dreams. But you have to be honest to yourself that the possibility of attraction is there, and if your girlfriend matters to you, take due measures never to let the scenes in the dream become reality. Reaffirm to yourself, in your head, that you care for her, and think of the other girl as only a friend.
As a guy, who this has happened to more than once, lemme tell you... It's natural! :p It takes a lot of effort for a straight guy to remain JUST friends with a girl, and never having had a thought about... Other stuff. :p I advise you not to worry though, as everyone feels this way at some time, and has similar dreams. Don't sweat it.