How to get refund from Dropbox

A family member tried to take advantage of a free trial offered by Dropbox.

While signing up for this free trial, they sent a lot of information. At some point, they asked for his credit card details which he duly submitted.

He subsequently found out from his credit card statement that he has been billed for a year's subscription as a business user of Dropbox. I agree he has made a mistake and he contacted Dropbox pointing out that he had accidentally signed up for a year's service when he only meant to try it out. He offered to pay on a pro rata basis for the service he had used so far and requested immediate cancellation of the subscription.

Dropbox has replied that no refund will be offered but he can cancel his subscription one year later. Subscription has been now cancelled to ensure no auto-renewal takes place.

Does anyone have any suggestion on how to get this refund?

FYI, one year's subscription as a business user of Dropbox is US$750 or almost HK$6,000.