tell her up front, there is no love interest. Honesty is best, sure she will feel the sting but it will pass quicker then her thinking more into what really is there.
My words exactly! :)
This is a common problem for those who are very attractive. Be careful not to lead them on. If they get a little bit too lovey-dovey, tell them that you cherish their friendship but that you're not interested in them romantically. But say this quietly, in private, and with extreme sensitivity. Don't embarass them. It may hurt their feelings to know that they can't have you as their mate but they'll get over it. We all must face rejection sooner or later.
Awkward moments are bound to happen.
Just be kind, OK?
She'll get that you don't feel the same way, though it takes time for someone to give up on that sort of thing. Be a friend if you can.
Treat her with respect and like you might treat anyone else. Don't encourage the feelings she has, but don't laugh at her either. And don't send any mixed messages like staring at her. Just act as if you don't even know about her romantic feelings.