It is like sharing a toothbrush. It might not be a problem, but we all carry bacteria and virus that may not be causing us any problems, because we are immune to that particular bug, or because we have good resistance to it. Anything from colds, flu, and sinus infections, to herpes, strep, mononucleosis, or mouth sore virus. Then there's just the aesthetics. I don't mind swapping saliva with my wife, but that's about it. However, different cultures and groups have very different attitudes about this. For some it is a big bonding experience to pass around a bottle of whiskey (or whatever) on the basis maybe that the alcohol is enough of a disinfectant (probably not). Some people think it is an insult if you do not share this group activity -- with booze, pot, food stuffs, and so on. People who have never had a good biology course don't even know about this as a potential disease vector, don't have a clue. I think you went above and beyond, buying your friend a cob and tobacco, and showed good faith despite your different concepts of personal hygiene. To explain all this would be taken as an insult to him, so I think you've done what you can.