Directions on how to cancel if within the 30-day trial period: link
More ("Note: You can only downgrade to the Standard Edition during the 30-day Premier Edition free trial. Once your free trial expires, all sales are final.")
EDIT: From Google: "If you'd like to cancel your Premier subscription after the free trial, you can do so at the end of your one-year subscription. As long as you don't auto-renew your Premier subscription, you'll have an opportunity to downgrade to the free Standard Edition." So, if it's past the 30 days, he has already probably paid. He will not get a refund, but will just have to wait for the premium account to expire. He will then be downgraded to the standard edition.