Hi,
i have smoked for over 20 years, and outside of when I was pregnant, I have never quit entirely successfully. However, I have been trying a "step down" approach this passed year. I was about a 2 pack a day person, now I smoke maybe 2 to 5 sticks a day, sometimes I skip a day. Much less shock to the system. Some vitamins and herbal cleansing remedies have not hurt either (see your local healthfood store).
My mom died from lung cancer in 1992, and I figure the risks of that are far worse than the risks of "shock"---he may try an herbal smoking cessation product, in the form of a homeopathic medication, which usually contains some trace of tobacco to ease the transition and detoxifying ingredients. The phlegm is going to come up, and probably for some time!
Herbal teas, hot baths or showers to cleanse and purify, lots of drinking water and light exercise--just walking--can really help. It has for me.
Good luck to you. R