I'd tend to agree with Hex here. From everything I've gathered, while John Green is by no means easy to contact (he gets such a massive amount of mail that he has long since made it known he simply doesn't have the time to read, let alone answer, it), it is possible to do so through various venues -- all of which are provided on his webpage. And he has long been known as someone who values his readers highly and treats them well, and tends to be quite supportive of them; also one who is very good at reaching solutions which work for all parties. In other words, someone who is quite conscientious about such matters. So, until I see something a good bit more substantive than I see here, I'm inclined to be highly skeptical about any wrongdoing on his part. The only fault I see is his not remembering where the line came from... and that sort of thing I've seen with any number of writers over the years, particularly ones who have done a fair amount of work.* (Moorcock, for instance, has frequently been nonplussed when someone quotes something of his until the source is cited; even then he doesn't recall it, but accepts that it is indeed in that work. Lovecraft, for all his intense involvement with his work, did not always recall something from a particular piece, either. And so on.) I'd say this was simply a case of such a lapse, combined with the sensitiveness of youth, and quite likely a fair amount of abuse from fans who simply don't like seeing their idol as having any flaws... even if he himself has always been up front about no few of those he is aware of....
*While he has not written a huge number of novels, he has been involved in an enormous number of projects dealing with education in literature and other subjects, as well as being closely involved in the adaptation of his work to other media; all of which can blur memory of any particular work rather quickly.
