First, regarding Zyban and any other medication - check your Ops Manual, check with your company AME (if they have one) or your own AME or GP before taking the medication. You won't necessarily lose your medical or be grounded, but you need to be aware of exactly what the effects are likely to be.
Secondly, as far as 'treating' smokers is concerned, you may be interested to know that the government is keen to improve the health of the nation and has produced documents called 'National Service Frameworks' to guide health professionals. There is one called the NSF for Coronary Heart Disease in which one of the 12 targets is to help people stop smoking. This involves setting up smoking cessation clinics in the community usually run by nurses. These have been trialed and have found to be successful.
You may find these statistics interesting - more than 1 million people suffer from angina, 300,000 have heart attacks and more than 110,000 die of heart problems every year in England.
Some of the complications of CHD include angina, pain in the legs while walking, chronic obstructive pulmonary disease, stroke and impotence.