Talk to her calmly. Don't rush in and start screaming, "WHY WERE YOU SMOKING!!" (etc.).
Instead, tell her you're concerned. Tell her that her life is too important to be destroyed by cigarettes–*that you don't want her to suffer like many smokers do after they've been addicted.
Also, show her this:
http://www.youtube.com/watch?v=zuh2w2sFRMI
And this:
http://www.youtube.com/watch?v=SfAxUpeVhCg