Catpiano

Dont ever try to fool the US/Canadian custom ..

Even most american/canadian are dumb ass ...  

but  ... as long as it is TAX related .. they are smart ass !  (oops..  ass anyway )

My parents and brother come here with british passport,  and just using that, not even bring their Canadian passport anyway ...   same as when going to US, then back to Canada ..  no tax at all, however need to pay $10 US or something like that ...

( then no tax in seattle, since I am albertian! lol )

Catpiano

Just use HK passport law ..  just to be safe ..

However, since your friend is from HK ... he / she won't be interested to shop in US/Seattle anyway la....  ( unlike us ..  Canadian ...  lol )