Well, depends where you want to buy from duty free shop in the airport or from Parnell, which is not far away from central city, got more selection than airport, but its going into winter now, so it will be winter collection. If you need more details just let me know, because my work place is not far away from Parnell Timberland.
You shouldnt worry about getting boat shoes for your friend, because Auckland has got a famouns name "city of Sail", which means there are alot of yacht that could be seen at Port of Auckland/waterfront.
There are fair bit of collection of boat shoes for man and women in Parnell, for sure there should be some available in the airport too, but not as much as Parnell, therefore i dont think you have to worry about it. But because Asian seems to have smaller feet, so it may be a bit hard if your friend's size is smaller than size 9. They can order from other shop, but it just takes time, if you are not staying here for long, may be it will be wise to get it in the Airport.
When r u coming to NZ? Today's temperature dropped dramatically, it was still 22 degree yesterday, but its only 14 degree during day time, 9 degree at night time. So bring alot of winter cloth, as we are going into winter now