HI, I live on Maui, and although I have not ridden to the top, I have done partial climbs on rides. I would suggest you have your client do climbing intervals to build stamina and endurance, also try to get some mileage at higher elevations. The summit is over 10,000 feet. That is 2 miles up. The air
gets noticeable thinner at about 3,500 feet.

I find interval work has helped me the most in building my climbing skills.
Another thought, would be to contact Go Cycling Maui and ask them for some input. They sponsor the Cycle to the Sun race each August and the owner, Donnie Arnault, is a former racer. They lead many cycling camps and also do rides "up the hill." They may have some targeted tips for you.