My opinion, its her life, she can make her own choices, and she is your ex. You have no control over what she does, alot of people smoke, my friends do, its their life let them get on with it, peer pressure is a load of bs because you make the final choice. however if she smokes near you you can say dont smoke near me, or tell her to go outside if shes indoors... Not saying you have to support her smoking, she knows the risks, its an expensive habit