Well last year she said hereself "If I didn't have a boyfriend, I would go out with you."
That sounds like something that is said as a compliment or to boost
your ego. It's like saying, 'you're a great guy and some day you will find some one.' It does
not mean she wants to be that girl or that she wants you fawning all over her.
Her relationship with her boyfriend is just that HERS not yours. In fact, trying to weasel your way into her life is nearly as bad as him supposedly controlling her. Both of you would be doing what you want instead of her exercising her right to make choices for herself. It sounds like she has made her choice. Respect it if you respect her.
Be her friend IF you are friends. Take off the blinders so that you can see the available females all around you who might be better suited to be a girlfriend for you. Good luck.