You cannot be this close to someone and NOT interfere with their decision-making process. She needs to decide for herself --- and she cannot do that with a 'friend' who wants her romantically, influencing her emotions.
You are playing a 'game'...and you are wanting the 'prize'... If she plays along with all of this, then you have to wonder... what kind of 'prize' have you actually won? A girl who is so easily swayed... who is too weak to resist the 'grass is greener' temptation.
You already have the makings of an unsuccessful relationship... should you happen to win her over. If she is swayed by you... then it is clear that she cannot be trusted to remain faithful --- that she cannot resist temptation. How she treats her current boyfriend is a good indication of how she may treat you if you were her boyfriend.
Back off and leave her be... if she wants to be with her boyfriend... then you have no right to interfere -- no right to nurse whatever doubt she has about the relationship. To do so is equivalent to manipulation and you will have had a hand in the destruction of what could be a functioning, healthy relationship. Something I'm sure you wouldn't want to have done to whatever relationship you find yourself in later on.
However, if you decide to throw all this to wind and pursue her anyway... you will find yourself in the same predicament as her current boyfriend in due time... reaping what you sow, despite your best efforts, manipulation, and false hopes.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen