No matter how drunk or not drunk the individual in question was, he was completely wrong and 100% responsible for smoking. The FA is not at fault here. He is.
Would it be a security risk? I doubt it. In fact, I remember that not too long ago the lav doors on the 777 airplane had a pretty obvious space to extinguish smoking material. It may even still be there. Sure, it is (now) unexpected, and one could question the inherent hazard of smoking on an airplane (and debate whether it should ever have been permitted). But what is not debatable is that tobacco smoke is an environmental hazard to everyone around the smoker, and in an cylinder where the air is recycled the stench spreads pretty quickly. For having seen what happen to air quality when pilots light up (in China), it seems self evident. That is the main reason why it was forbidden and why this type of offense needs to be treated seriously.
Seems like he got what he deserved. Excellent!