There is no saying that you did put the idea into her head. And if you did, then it is possible that she would have started it anyway... When I started SI, I had no idea that anybody else did it. I thought I was the first person to think of it, lol. It was only when I joined online communities that I realized the truth.

I think at the moment you're handling too much pressure, especially now that you have the threat from your friend hanging over you. Can you refer her to anybody else she knows? Suggest to her to tell her parents, or perhaps for her to get a counsellor? You could give her some support sites for cutting maybe, or just write an email or a letter pointing out that cutting has a tendency to create more problems and isn't beneficial in the long run...

Right now, you're just not coping with all your pressures, and you have to put yourself first at the moment. Congrats on not cutting.