She sounds like a moron, but it doesn't sound like pictures, discussions of the consequences, etc will do anything, because she's in "won't happen to me" mode. I strongly advise you NOT to throw out her helmet. No matter how 'right' you are, I know I would react violently to someone throwing my stuff in the bin because they thought they knew what was best for me. That's seriously presumptious and condescending. Even though I agree with you 100%, don't go there. All you'll probably achieve is a pissed off friend, and perhaps theft charges if she's really pissed about it.
OTOH, there's nothing stopping you reporting her to the cops.
