Well, you could always point him at hbpspork--where the bad points of the book were sporked--chapter by chapter, as if it were a fanfic--and the good points (considerably less, alas) were praised. (We tried to be fair.)

He might also like Mike Smith vs. The Half Blood Prince (http://pages.prodigy.net/mike_p_smith/hbp/intro.html). You have to love a guy who refers to the character of Harry Potter as "the Rowling Family College Fund." (However, HBPspork is shorter.)