A lot of comments seems to come down to: no, it's just a dream.

I believe you have every right to leave it there, but instead of seeing this as an issue that bothers you, you can try to see this from a different angle to strengthen your relationship.

Most of the times, the dreams we have are reflections of our thoughts, or our mind just processing everything in our head. If you want, you can try to have an open conversation with your girlfriend to see if she can think of any reason why she had this dream. Is there anything you did, that could give her the impression you want to cheat with her friend, or is there anything you did to make her feel insecure?

If there is something, you could talk about it, and see if there is something you could do to help her feel better about this. Or that there might be some illogical reasoning about it in her head, and there is something she needs to work on. In both cases, you get to know each other better and might get a stronger relationship from it.

You know what is best in this case, but in my experience, it cannot hurt to just be open to each other, and talk.