Taking care of one’s emotional health/psychological well-being is more important than taking care of one’s physical health. Of course, ideally one should be able to do both, but most people do find that to some extent they have to choose between one and the other. Most people are not “holding by” living a completely healthy lifestyle and it would break them emotionally to try to be “perfect” in this area.
For example, I know of a lady who was suffering from depression, and she dealt with it by smoking.
You never know what choices other people are making, and we have an obligation to judge them favorably, just as we do in other areas. Maybe the guy you see smoking just got out of rehab and is trying not to have a relapse.