I have to say that I'm kind of surprised no teacher has caught onto this. I mean, I can see a creative writing teacher not using turnitin, but most cheaters are not nearly as clever as they think they are.

As a teacher, I naturally would want to be told about this. But I also recognize the social quandary. If it weren't for the curve question, I'd honestly (npi) advise the daughter just to stay out of it. (Side note: I hate curve-based grading. Haaaaaate it.) If she can get her peer group to stand with her, then she should bring the matter to administration. If she can't, perhaps there's a teacher she can approach for advice without revealing the cheater's name? The anonymous approach isn't really going to work, given that the daughter has already confronted the cheater; cheater will know who did it.