I tend to think that not being judgemental is best. Maybe there are exceptions, but I don't see that criticizing someone else's choice of religion, diet, life partner, etc. helps us cope with cancer.
As for a 15 year old who smokes pot and is "paranoid" about his cancer returning, let's look at the "paranoia" first. Is a fear that cancer might come back really paranoid? I think most of us have that fear to a greater or lesser degree. It is entirely rational. It is important to deal with the fear and get it under control, but calling someone paranoid for having the fear does little good. Helping them cope with the fear is better.
And then there is the pot. OK, it is illegal in many states and only vaguely legal in others. I don't think this is the proper forum to discuss the legal aspects of pot. Many people report that it helps them with pain and with certain medical conditions (glaucoma for example). I don't know why the kid was smoking pot or the specific legal situation where he lives, but it is possible that pot has an effect on his fears. So, I think it reasonable to suggest he discuss his fears and, depending on the legal situation he faces, his use of pot with his doctor. Yes, smoking anything is not good for you. I don't see how pot smoke is any less carcinogenic than tobacco smoke.
But, condemning the kid for using pot is probably counterproductive. It's a good way to turn the kid off and ensure that he does nothing to improve his life or his health. You can make gentle suggestions. Suggest that he find another way to control pain, etc. if that is why he smokes it. But, demanding that he quit immediately does no good. Maybe he should quit immediately, but a gentle approach will be more likely to succeed.