Don't date her, don't show interest in her, don't lead her on. She's in a relationship and is incompatible with you. Just chat as a friend. See if she has any hot friends you can go for.

__________________
My Demon revokes any prayer
He's grown contempt for love and hope
He betrays trust, twists truth and fair
Indifference is his way to cope
Engulfing sound of sensations
He quells with voices of despair
And muse of short lived inspirations
Flees at the sight of his cold stare
~Moy Demon - Mihayeel Lermontov~