Have you and the 1st BF discussed this with each other- I can only assume so. She may be over compensating with him to ease his feelings...(??)
One thing that I suggest is to promote (actively while you are all together ) the intimate contact between her and him and likewise between you and her while he is present. Voyeurism is definitely not everyone's cup of tea, and intimate contact doesn't always have to mean sex, but it sounds to me like she is afraid of distancing one by loving the other...
I completely understand your feelings- I have been exactly where you are and on the other side of the coin as well. It might sound a little cliche but some open communication might be just what you all need to clear the air.