I think there is no absolute right and wrong in this situation. It comes down to the relationship, and the feelings of the people involved. She has a right to tell him that what he is doing is unacceptable to her. He has the right to tell her that for her to control what he does when he isn't around her is unacceptable to him. What it comes down to is whether it is so important to both parties that no compromise is possible. If that is so--if he feels controlled and untrue to himself by refraining from smoking, and if she feels disrespected and threatened by his continuing to smoke as he does, then the only solution is to split up. If the prosepect of that causes either party to change his/her position without doing violence to their own principles, then that's what happens.
What would be awful and destructive would be for one party to give in, while seething with anger and resentment, or for this issue that has become so important to be the subject of lies, deception, and suspicion.
There are healthy and unhealthy ways of responding. That's more important than what the ultimate solution is.