I had a friend in college who was in a totally toxic relationship (not an age difference, but I daresay I was just as worried about her). I finally came clean and told her that I thought she was making a horrendous mistake. She and I fell out over it...had a hard time maintaining the friendship over the next several years. But when they did finally break up (the guy was a class A asshole), she said that she appreciated my being honest with her...even if she didn't like what I'd been honest about.

What am I trying to say? If this is your friend...and you care about her....and you think this could really truly be a big time mistake....you're gonna have to go with your heart. If you get nowhere talking to her, and you think the threat is serious enough to go to her parents...do that. But realize that it might not have the result you want. The Romeo and Juliet syndrome is a very real thing, particularly at her age. And it may alienate you from her. In my case, I was worried that she was going to be physically as well as emotionally hurt....it was worth jeopardizing our friendship. Still, I wish I'd found a better way.

Sorry I can't give you more simple advice....hope you find a way through the morass.