Having a BF doesn't mean jack shit. I've had *** with girls who had/have boyfriends constantly. Though the majority of the girls that put out, and flirt even when they're cuffed are thots.
But really Boyfriend is only a title, it doesn't mean shit honestly. She'll switch up sooner or later when she sees "better choices" (obviously talking about men).
So. Just flirt, and do whatever you do to get in a single girls jeans, and apply that to this taken girl. Shouldn't be hard unless her boyfriend is better looking / endowed (lmao) than you. You did say she likes you, so go for it champ.