Is a heavy smoker much more likely to develop cancer than a lighter smoker? my friends says "it doesnt make a difference how much"but that seems false

You are correct: It's best not to smoke at all, and it's the nature of the addiction gradually to increase. "Teach abstinence only" may be politically correct, but it's simply not true that an occasional smoker's risk is equal to a heavy smoker's. Risk is measured in pack-years (packs-per day times years smoked) and relative risk is a pretty good function.