As others have said, there is an outlet mall in Lahaina. If they are staying at one of the Kaanapali area hotels then there is a cheap bus that goes there or a short Uber ride. There’s Coach, Tommy Hilfiger, Banana Republic, Adidas, etc. as well as a store called Hilo Hattie’s that sells Hawaiian clothes and Dole Whip! The mall is on Front Street which has lots of smaller boutique stores and restaurants as well as the second largest Banyan tree in the world which is incredible!
While it’s a must-do for me, coming from Australia, I would still suggest hitting the shops after the sun has set on the beautiful beaches and happy hour has finished; they close at 10pm so there’s plenty of time!