Different islands in HI have different tax. If you're friend is going to Oahu, why don't you just contact the LV stores in Oahu and ask the price/availability of the item that you want. They can also tell you the exact tax rate for their city.
I went to Maui last year and called ahead to verify the price of the Damier Neverfull and tax rate. The SA offered to hold the purse for me since I was arriving in Maui two days later.
Just make sure when you contact the Oahu LV store that you are talking to an SA that actually works in that Oahu store. Sometimes when you call LV, the main 800 number phone reps pick up the phone lines for the individual stores, so you may not actually be talking to someone at the actual store.
HI pricing is cheaper than continental U.S. pricing since Hawaii is considered a separate region. And tax rate, I believe, is in the 4% range but depends on the HI island.