Hi and Welcome! There have been times when I've dreamed things very similar to people I've lived with or worked with. As we look back on the images, we realized we were exposed to the same thing, either a movie or somthing in the news. For example, many years ago my mother and I went to see a film called "Bad News Bears." The next morning she told me about a dream where a black bat got ahold of her fingers and wouldn't let go. I told her about a dream I had that night where a black panther got ahold of my arm and wouldn't let go. We laughed at the coincidence and figured it had something to do with the movie. Is your girlfriend pregnant in reality? If so, it could explain the dream about kissing her friend. If the sex part of the relationship is on hold, then it could be wish fullfillment. At least you are cheating in a dream and not real life. That could cause bigger problems than disturbing dreams. ;-)
Help you? Actually what you describe is quite normal. It's something called hormones. Relax. Nature will take its course when you're both ready.
I would say you should really be certain before you step into anything, life experiences are coming your way.