Sylver,
I can certainly understand why you would feel so bad about this

Please try to remember that it isn't your fault if she does it, though. I have shared with several of my friends and none of them have cut as a result (at least not that I know of). This might sound a bit fatalistic, but honestly I think for those of us who become cutters, if we didn't see or hear about it from one place, we'd hear about it somewhere else and I think a person who is going to try it is going to try it eventually. Not that anyone is "fated" to cut, but there are certain characteristics that most of us share which very much contribute to our becoming interested in (and later addicted to) cutting ourselves. If your friend didn't hear about cutting from you, she might have seen a show on TV about it. She might have stumbled across it in a webpage. Met someone else who cuts. Read about it in a magazine. Or even eventually just thought of it on her own.
Even if that weren't true, it is still her decision to pick up that blade if she does it. NOT YOURS.
I don't think she was joking. I think she was either seriously suggesting that she thinks about doing it, or making a jab at you. I would think it would be the first thing. (If not, if it's the second thing, lose her because she's a bad friend).
I don't know if this helps at all. I sure hope it does. I'm sorry about your friend
*big hugs*
Angela