No, you can't change the email and let him change the password.
Accounts are not transferable per the Terms of Service.
http://www.graalonline.com/docs/agreement.php
13.General Provisions. You may not use, copy, modify, sublicense, rent, sell, assign or transfer the license granted pursuant to this Agreement except as expressly provided in this Agreement. Any assignment in violation of this Agreement is void.
End result will be in a ban of the account.