Do you think I should tell my friend's girlfriend that he smokes weed?
Why would you tell her that?
I think it's a problem, and he doesn't even know how to smoke weed right yet.
It's not right to go behind a friend's back like that. You should respect your friend's personal life.
I just don't like him being a pothead.
Try speaking with him directly instead of going behind his back and telling his girlfriend. It's good that you're concerned about his drug use, so maybe you could offer to help him get some help overcoming it.