RE: Would you turn him in?
If a friend was breaking other laws would you turn him in? Say, if you knew a guy was stealing from stores and was not going to stop or if you knew he was drinking and driving and wouldn't stop? I think the answer goes beyond the breaking of game laws and speaks directly to knowing someone is breaking the law, any law, and either ignoring it or doing something about it.
Arthur's answer pretty much sums it up. Try and reason with him but if that does not work then you need to decide whether you should allow him to continue breaking the law, any law, or doing something to stop it.