I don't think you can do anything yet. If you try to tell him not to, then he will just get upset and do it anyway. Just warn the fellow and let him know that you are concerned. He has to make up his own mind from there.
Some people need to hit rock bottom before they realize they are in a bad way. Sadly, sometimes they don't get a second chance.
I know someone who has had their entire life slip away from one night of taking meth. He almost killed a few others and he is now locked away. He was only 18 too. He didn't get a second chance.
Actually, there is someone who can do a lot to stop it. His parents. Have his father give him a good swift kick in the ass.
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