A recently completed Cochrane Collaboration systematic review led by the Schizophrenia Group, was unable to come up with enough high quality evidence to indicate whether professional advice provided to people with serious mental illness is effective in reducing smoking among that population. However they did point out that people with serious mental illness are about three times more likely than the general population to smoke.
There may be a direct causal link between mental illness and smoking or it may be that smoking helps people deal with the stress of mental illness, or helps with the side effects of medication, such as tiredness, drowsiness, and boredom, or both. (cochrane.org)
Seems like exploring this question might be more important than their original one.