What would smoking do to a person if they had brain cancer?

It depends: Based on life expectancy i might or might not advise a patient with brain cancer to stop smoking. In any event i would suggest nicotine replacement to avoid some of the risks/hassles of tobacco use. Smoking becomes a big hassle in clinics, hospitals, around oxygen, etc...So all the more reason to transition to a patch or nicotine inhalers if possible.