Only the user can convince himself to stop using. You may transmit useful and important information to such a person, but when or whether that information is used by that person is solely up to them.
Also, do not confuse a habit with an addiction. An addiction is a very specific thing, and that thing is not caused by marijuana use.
A habit is far easier to break than an actual addiction, because addictions have serious and sometimes lethal withdrawal symptoms.
Just because someone does not chose to stop smoking doesn’t mean that they can’t, or will suffer greatly if they do, it just means that they do not currently want to.