You can tell her at the right time, because I also experienced that before. I’ve been in love with a guy friend and he has a girlfriend. When i told about my feelings with him, he was really shocked about what he was heard from me. I know he was shocked because he’s not expecting that I have a feelings for him and he treat me just only for a friend. It really hurts my feeling, when the one I love treating me just only for a friend. So, I would suggest you to make the right decision. If you’ll know that you a chance, then go!fight for it! 😉