A lot of assumptions and projection on this thread!
OP, have a talk with your son - just the two of you or also his other parent, if s/he lives with you. Tell him you smelled cannabis on him and it needs to be discussed. If he smokes regularly or is often around people who do, he may be "noseblind" to the scent and genuinely not realise how immediately recognisable it is to non-smokers, even the traces on clothes and hair.
If he says he doesn't smoke, he only tried it once, girlfriend had a few puffs but he didn't, they ran into some friends who were smoking but didn't smoke themselves, etc., don't argue. Since you've never discussed this before, your goal is not to punish him for whatever he's already done, but to make sure he's aware of the risks and to make it clear what your rules are - for example, no smoking in the house, no one brings drugs into the house, etc. (And if he is smoking, try not to blame the girlfriend - even if she smokes like a chimney, he can choose not to join in.)
As far as your friends' children, it doesn't really matter what they do and no one here can know. Cannabis smoking is common among some groups of teens, others consider it gross, unnecessary, or passé. Since it seems like your son might be smoking or interested in smoking, the best you can do is get informed about the downsides and risks and get the topic out in the open. Good luck!