You like her. She's dating him. It may seem like an impossible situation. But remember, dating isn't married. The younger you are, the less likely it is that her relationship will last too long. The best thing you can do is be the best you possible, be the best friend to her possible and be patient. And if an opportunity arises for you to let her know how you feel, take it. Knowing your feelings may be the only push she needs to leave her current relationship and give you a try.