I totally agree about matching a peroson with the board designs own suggestions, I personally would put him on a 152 (the shortest size they have) - but heck if he really is as good as he says he is (those Vancouver racers are no joke) and going really fast... the 160cm is going to be tough, but it is not going to be impossible for him to ride. Once you are at expert level... board lengths are most just suggestions (I doubt you would have recommended for me to ride a 178 cm board
at 140 lbs either).
After the last few days, I'm tired of arguing with people when they don't want to listen. He is really that determined (or that stupid) I say let him do it. Either he's going to have a terrible time
(and learn an important life lesson
)... or he's going to be alright. Eitherway its going to be entertaining.