stop being a white knight and grow a pair. It is her choice whether she wishes to smoke or not. She's probably a grown woman and you're neither her boyfriend nor her father. Also since you don't like her smoking, stop being pussywhipped and stop buying her cigarettes. Tell her she can go get them herself if she wants one. Bring up the topic just one time and warn her of the harmful effects of smoking. If she doesn't drop the habit, drop the topic and don't bring it up again.