She has a boyfriend- so you know she's hetero. Do NOT tell her- you will weird her out and lose her as a friend.

Have you fantasized about being with her sexually? Or maybe you just adore her company so much you don't want to share? Secondly, have you ever been with a woman yourself? If not you may just be going to a sexualy confused phase, understand?

I'd just wait it out and see if these feelings pass.