Here's how making a move will play out, with almost certainty:

If she likes you, expect a few weeks of her being undecided over you or her boyfriend. This will be emotional hell for you, as you do not know the outcome. But I do. She's not going to leave him. You know why? Because she knows him. She's safe with him, and unless he's fucking her over or knocking her around - and face it, even if he was, the odds of her leaving him are slim - she's not going to leave him. Meaning, she's left you hanging out to dry while she 'mulls over this decision'. Every night, you'll go to bed thinking about how tomorrow, you'll get your answer and it'll be nothing but sunshine and puppydogs licking your balls until you're blue in the face, but it's not going to happen.

Nut up and move on.