As a smoker, here's my two cents, whatever it's worth:

My husband, when we started dating, hated that I smoked. But I wasn't naive enough to promise him that I would stop. That's just not realistic! It's not my conviction to quit, it's his. You never know, she could have just quit smoking around you. If it's not her decision, then what's stopping her from smoking behind your back? You can't make anyone change themselves without their consent, and point blank, she's not giving her consent. And it is rightfully her decision, not yours. I see her point in seeking independence.