My dad has been smoking for over 30 years now. What is the likelihood of lung cancer?

Tell him to stop. If he stops now, then his risk of developing lung cancer decreases.
Increased. His risk is about 30x that of someone who has never smoked. If he continues to smoke, he will have a 1 in 10 chance of developing lung cancer. It is never too late to stop. Family, friends, and doctors can have an influence so continue to urge him to stop.
Possible. As noted, first thing is stop. Work with family physician, get help, and stop. Second, it's very important to understand there are numerous disease that smoking will increase risk. Lung cancer alone, there are some online resources to help calculate risk: http://goo.Gl/divhj and http://goo.Gl/r9ita.
Lung Cancer is only. One problem, emphysema copd, heart disease, and other cancers. He has a 50% chance of dying of smoking related illness. Undoubtedly he is a nicotine addict, and will only stop when he wants to. Many more aids to quit than when smoking was first linked, but the key is wanting to stop, seeing the error, resting control back.