That is quite a humorous question; I like that you have named the nameless narrator "Bub," and referred to the weed that they are smoking by its more scientific name of cannabis. :)
Up until the ending, nothing impressed this rather bitter, whiny, and sarcastic "Bub". Of his wife's ex, he writes, "Her officer—why should he have a name?" Of his wife's poetry: "I didn't think much of the poem." Of the blind man's tragic loss of his wife: "Pathetic". Of his wife's attempted suicide: "But instead of dying, she got sick." This is a man impressed by nothing, cynical about everything, and pretty unpleasant. The fact that drawing cathedrals with Robert was "like nothing else" in his life up to that point is made all the more significant because of how unimpressionable the man is. It makes the ending more profound, touching, and effective. All of a sudden, through Robert's guidance, he CAN draw it, he CAN help Robert to see what he saw. He is drawing roofs, buttresses, windows, the whole thing, when before he was stuttering and felt pessimistic and incompetent. It is so moving that he closes his eyes, and without even looking at it, concludes, "It's really something." Robert helps him to get outside of his shell, to experience something out of his comfort zone, and finally to trust himself and his own capabilities. He opens the narrator's eyes to his own potential, and to beauty that exists within himself.
So, how does the cannabis play a role in it? It probably served to relax this rather uptight narrator; if not under the influence, he might not have ever let a near stranger take his hand and help him to draw something on paper. That is a rather uncomfortable experience, especially for two men, but because "Bub" was so relaxed and easy-going as a result of the cannabis, the experience was had. I'd like to be optimistic and say that it didn't influence the coolness of the actual moment for the narrator though; I'd like to think that he just realized how cool it can be to let your guard down, to let someone else into your life, and to see things from someone else's angle. He might not have had the experience otherwise. It can probably be argued either way though. Good luck! It's an interesting question. I hope my thoughts helped.