Who REALLY wrote Harry Potter and the Deathly Hallows?

I haven't read any of the Harry Potter books, but E, who read each book 2 or 3 times and in at least 2 languages says that the last book (7th, is it?) is so unsatisfying that she thinks she might have purchased a fake copy...

She went to the bookstore to find out (after having heard on the radio that the novel should be 764 pages long while hers is 670 or thereabouts.)

All books in the bookstore were the same and this got E to think that:

• perhaps this was just a joke to get everybody talking and the REAL edition of the 7th Harry Potter is still coming

or

• perhaps the book was not written by JK Rowling at all... E says it did not have the depth of the previous books and it did not add anything new.

Now, E wants to know: Who REALLY wrote the last installment of Harry Potter?

Reading Harry Potter at sunset