How can I let my friend tozzy down gently?

Basically he fancies a girl, we'll call her rosie (it is her name after all) but the problem is she clearly fancies me, the way she ignores me and doesnt acknowledge my existence, clear signs of attraction right? she also has a boyfriend.........what can i say to my friend? hes never had a gf before and thinks he has a chance and will be so upset when he finds out the truth