Come on kiddo... when you know that she is in love with some one else then why do you want to be the second guy in a girl's life? As you are asking the question here that means, you are still in doubt and you are not sure about her other affair. If you are sure, then why don't you confront the same with your girlfriend? If you are not sure, then just stop worrying. It might be so that you are just very insecure about your relation and unnecessarily doubt the commitment of your friend. So, chill out and choose one... CONFRONT with her OR LEAVE the matter to take its course.
Now cheer UP!!! :)