Going to be perfectly honest here. She very well might see it as just jealousy & hypocritical of you. I mean that was my first thought.
You're a decade older than her. Why is it your business that she's having "lewd/sexy" writing sessions with someone four years younger?
How realistic is it really that she'd get in trouble for this? What would the worst case scenario & how likely is it?
If you want to bring this up just be honest. "Hey I saw this. I know it's a reach but he is 17 so just be aware you're taking a risk. I'm just putting it in your mind in case you haven't thought about it. Could be worth holding off a few months, but that's your business."