It's probably a hoax/lie... read it hours ago.. doesn't have any proof or anything, and if he has the book why not leak it? or at least copy the end? quote a couple of sentences? because he doesn't have anything, and if he makes anything up, people will know it wasn't JKR straight away
Rumors like this are always there, and they don't say (publishers) whether it's true or not for free publicity(gets people curious), and if an actual leak happens, they won't have to say a thing....