Cannabis does not cause cancer. Inhaling carcinogenic smoke causes cancer. There are plenty of methods of ingestion that make smoking irrelevant and at the same time your friend's argument.

cannabis != smoke
smoke != cannabis

If he/they can't get over the smoking aspect to actually discuss the facts then you're just being preached to by someone who has been forced to watch DARE ads their whole life. Consider the source, have a good laugh, and move on.