Recruit a friend, Fail.

Hey all,

I've sent my husband a 'recruit a friend' offer to play WoW. He's accepted and created a battlenet account.
He's tried to enter the authentication key, which he received in an email, to get the game trial started, but instead has received the message;

The authentication key you entered cannot be claimed. Please check the key and try again. Alternatively, you can enter this key for a different region by changing the leading part of the www.battle.net URL (ex. “eu.battle.net”) in your web browser, or for a different Battle.net account.

Could I get some insight into why this is? Is it because we share the same home address for our accounts that something isn't working?

The key to type in has been checked to death and all his details have been entered correctly.

Help would be awesome. Thanks all.