For non-smokers, second-hand smoke exposure causes lung cancer. That's one of the reasons why we need to continue efforts to reduce exposure to second-hand smoke as part of the overall effort to combat lung cancer.
Also, once cancer is detected, it's very important, as part of excellent treatment, to provide smoking cessation assistance. Whether a person with cancer smokes or not affects their survival rate. The 2014 U.S. Surgeon General's report had an extensive evidentiary review for the first time of how important not smoking is to cancer survivorship.