Could you break into a coughing fit the next time he starts to smoke at his back door? Lean over the fence and in between the coughs say (with difficulty) "I've been meaning to ask you whether you could stand over at the other side of the garden when you come out for a smoke. Since I've had the lung problem, this is how smoke affects me, even if I am not at the door. I just can't breathe properly. It seems to go straight into any windows or doors that are open, and I can't keep them all closed in this lovely weather."