What should average weight for 6ft male? I have tried to say 17-18st is a severe health risk esp. To smoker that drinks 40 units alchol x1 day wk min?

Weight least of it. As far as his problems. 40 units of alcohol per day is alcoholic drinking, causes organ damage and life problems for virtually anybody. He needs help. As far as weight, for a 6 foot male, a weight of 180 pounds gives a bmi under 25, which is the lower limit for overweight. 17-18 stone (a british measurement not used in us) would be as much as 250 pounds and is obese, and is a risk to health.
Healthy weight. You can calculate this by using bmi (body mass index). Body mass index can be healthy between 18 and 29.9; a bmi of 25 is a typical, average, healthy figure. For you at 6 feet to have a bmi of 25, would imply a weight of 184 pounds. Before you worry about your weight, however, I would urge cutting down on smoking, much more serious health risk. The alcohol is also adding a lot of weight.