According to many rabbionim now days smoking is a major issur may even be a issur desorsah to start. Quiting once you started is another issue I went to whole shiur on problem of smoking today.
Many boys I know do not smoke and the ones that do a lot of them try very hard to hide from there wives or tell their wives they are going to quit. I personally believe smoking in a way is issur desorsah because you cannot purposely cause yourself harm.
In my opinion you should not tell the girl that he smokes but he should be willing to admit it if she does ask. Because I know many guys would deny it. It beginning trust matters a lot.