In short, bad things!
I'm not aware of any studies of the effects of a single cigarette smoked by a "non smoker". However, there has been lots of work done on the effects of chronic smoking on diabetes and glucose metabolism. In short, smoking reduces insulin levels and increases insulin resistance. This means that smokers are likely to find it harder to control BG levels than non-smokers. Heavy smokers (non diabetics) take significantly longer to clear a glucose tolerance test than non-smokers. Add to this the increased risk of heart disease and strokes associated with smoking and you see that adding those risks to the risks of diabetes is really asking for trouble.
I remember reading somewhere that the majority of people with really serious diabetic complications (amputations, total blindness etc.) are smokers. I can't remember where this was, so I don't know how reliable it is, but given what I know about the metabolic effects of smoking I don't find it hard to believe.
Although I know that it is hard, all smokers should really try to quit. For diabetic smokers the compounded risks really should concentrate the mind.