if the girl smoked before you met, this should become a sign for you that you are already creeping into her life under the pretext of your wishes, you do not need to change the person for yourself, then you will not like it much
if when she started smoking you were there, why didn't you say right away that you were against it? you have only one option - to talk, ask if she is ready to stop, but if suddenly she is not, then she does not have to, you have to understand, maybe this is the very moment where you need to give in?
You can try to beat her ass with fists with a wild cry, but personally I advise you to tie and extinguish cigarettes on your nipples. I hope you succeed, good luck in your relationship!