Re: Harry Potter - Book 7 Theories
I don't think that the characters in the book that was published are those of Harry Potter. According to the article linked above, this new novel is one of a series of three that "centres on a mysterious series of suicides amongst young people in the fictional community of Puerto Fake." Random House is a pretty big publisher, and I doubt they would publish anything that resembled Rowling's work enough to generate a plagiarism lawsuit.
Fanfiction, though, is written for the entertainment of other fans of works of fiction and are not generally done for profit. If you have a look at the site fanfiction.net, you will see the vast body of fan fiction that is written but not for profit. It looks like this young woman wrote something on her own to counter deficiencies that she found in Rowling's work. Good on her though - I can't finish a 50-page paper for a Master's degree, far less complete a 756-page work of fiction for people's entertainment.