Oh young lost soul, what will we do with you. I don't know whether to kick you or stone you. Your love for this girl is not a healthy one, to want something that everyone else has already had leaves one to question your sanity. I'm sure she has cried to you, promised you things, she probably even gives you goal dates as to when she will leave him. Sound Familiar?
Most of us have been down that road, so let me tell you my story. I spun my wheels in this exact same situation with a man for over three years, by the time he did leave, only because he got caught, I had not respect left for him and the relationship didn't last a year.
Have respect for yourself and walk away from this mess. If she truly loves you she will see your efforts to move on and if she wants to be with you when you move one, she'll leave her perverse world, of minipulation, lies and pain. Bad thing is, she'll be bringing all of those bad habbits to your new relationship. The only way to keep this one happy is to trade in your spine for a pair of rose colored glasses, because your going to need them.