I remember we came down pretty hard on DS #1 when he told a friend of ours "you shouldn't smoke!" But in all fairness, he'd learned it from Captain Kangaroo and we hadn't yet had a chance to reinforce the message that little people don't tell big people what to do, as well-meaning as they might be. And he was only about 4, which is very different from 8 or 9 as in the original letter.
As PPs have said - if it was truly an outdoor non-smoking zone, the child could have politely informed the letter-writer about that. One thing that I think some of these anti-smoking/anti-drug programs neglect is that they don't inform kids that once people start smoking - stopping is very difficult. Of course it can be done, but many adults who smoke have already tried stopping several times - and even if they still smoke, they might have cut down substantially from the amount they were previously smoking. No adults in the developed world who smokes can be unaware that it's not good for him/her - they don't need a child, especially a stranger-child, to tell them that.