Dude, not always you have to go with the manual, depends on the kind of music you like, DD recommends 35 hz tuning and i tuned to 41 hz so i didn´t lose the highs, more than happy with the result!! Just because Fi says tune to 32 hz doesn´t mean everybody listens just that kind of music, besides that, if he get the port that big (talking about the length), he is shrinking the box, would give him around 11.5SQ of port per cubic feet, maybe the 12SQ of port, but i prefer a little bit high on the tuning and some more output with 13.5SQ of port per cubic feet, but that´s just me of course.
Said all that, it depends on his music likes of course, not ours, i can asure you that my next box for 2 18" it´s gonna be tune around 32 hz just like you told him