Let’s suppose he discovers that his chavrusa was surfing the internet for schmutz. Would you then approve of his being dropped as a chavrusa? The smoker is violating open halachos about “Venishmartem me’od lenafshosaichem”, and this has been stated openly by scores of Poskim and Gedolei Hador. There are numerous threads here about the issur of smoking, and repetition is useless. The point here is that one may choose to drop a baal aveiro as a chavrusa.
If one does not care about adhering to mitzvos, as is apparent in his choice to continue smoking when the world’s greatest poskim declared it against halacha, why would one respect that individual’s Torah that he chooses to not follow?
I will yield to others to post the links to previous discussions about the halachic aspects of smoking.
Lastly, I have read the gibberish from those proponents of smoking. Every single one approaches the issue as something that “must be muttar because I wish to do it with impunity”. They continue on to twist logic and words of halacha psukah to meet their predetermined goal of brandishing a heter. This is a form of self-bribery, and is not halachically sound.