Can't argue with your friend, but just wondering if you ever did any research about those saying the opposite? It might be interesting for you, hearing first hand from a friend, to see what other people who are supposedly researching the exact same thing came up with.
He's in the field. I trust his research and logic. And think about it. If pot generates more tars and carcenogens, it's only naturall that similar cancers would result. That isnt rocket science