It is no chumrah to refrain from smoking. it is being oveir on a Lav d’Oraisa if one smokes. Period. I could also make a good case for a person being oveir on “Lo sa’amod al dam reyacha,” if one knows his chavrusah smokes, and does not dissuade him. Unless he is encased in a glass bubble, the smoker afflicts himself AND anyone around him. You want to commit suicide – please DON’T. But whatever you do to yourself, do not do it to me, too. ( I am not referring to anyone on this forum, because none of the intelligent people on this forum would dream of smoking).
Also, to address the comment that maybe the boy could hide his smoking from the girl… no way could that happen. Smokers truly do not realize how much their bodies absorb the odor of smoking. He would have to have not smoked for HOURS, be wearing pristine clothing that was not exposed to the smoke, and have had his teeth and nails whitened for the occasion. A smoker looks and smells like a smoker, all the time. They, like many dog owners, become inured to the odor after a time, and don’t notice it at all. BUT WE DO.