This is sticky because while it is still technically illegal, in most states, it's not frowned upon like it used to be. I think the notion that "Mary Jane" leads to harder stuff is just a scare tactic. I would stress responsibility, minimal use at home only, point out the health benefits and downfalls, and don't use it for party purposes, but for relaxation. In other words, be an adult about it.
I have a daughter so I have put a lot of thought into this because drug use is rampant these days. But really, it all comes down to your personal views about it. The only thing I can really suggest is that you don't turn the discussion into an argument. Keep the line of communication wide open. That's the main thing.