I don't have much to say here, because Cancer Research UK have already written a winning answer. (Sometimes the giants are tall enough that there's no need to stand on their shoulders.)
If you visit Tobacco, smoking and cancer: the evidence you will find explanations of all these facts:
- Smoking is the single biggest cause of cancer in the world
- Smoking greatly increases the risk of lung cancer
- The people with the highest lung cancer risks are those who:
- smoke the most cigarettes per day
- smoke over long periods of time, and
- start smoking young
- Smoking is a major cause of several types of cancer
- Stopping smoking can reduce your risk
- Tobacco smoke contains many dangerous chemicals
- Tobacco smoke contains significant amounts of dangerous chemicals
- Chemicals in tobacco smoke can build up to harmful amounts
- The chemicals in smoke are more dangerous in combination than individually
- The poisons in cigarettes can affect almost every organ in the body
- Nicotine is a very addictive drug
- Smokers are still exposed to dangerous chemicals if they smoke filtered or ‘low-tar’ cigarettes
- Alcohol and other substances worsen the effect of smoking
- Second-hand smoking also causes cancer and kills thousands of people every year
- Children are especially at risk from second-hand smoking.
- Smoking while pregnant can harm your baby
- Smokeless tobacco can also cause cancer
They explain it all clearer than I could hope to, and include references to 93 different journal articles. You've got to admit they've certainly done their research on cancer, those guys at err.. Cancer Research UK.
Oh look, a donate button. Maybe you can persuade your mistaken friend and his father to donate to make up for all the people they have misled with their ignorance and semantic word games?