Good for you for being proactive!
Be prepared for his being resistant to letting her go. She is apparently already very possessive and manipulating. This is not going to be easy...but you know that.
One of the worst detriments to poly (in my over inflated personal opinion) is people who proudly proclaim the label but don't really do any research or self education about what poly really is.( I am not casting judgement as I have had my own personal humbling as I started out on this life). If the girlfriend is calling herself poly yet acting as you have described...then she fits into the above category.
I applaud you for searching for more information. Perhaps your hubby would be willing to do some reading or be open to attending some sort of poly gathering? Having some form of support is always a great idea.
It is sad that open honest communication isn't the foundation of the new relationship. She sounds like a cowgirl....another new word that I first heard of here on this board.