Give me a time, a server name and a char name when you/he tried to connect, I'll check the logs when I get home.
Odds are he bought his key from cdkeyhouse or direct2play, two stores that sell their keys to multiple people.
Since all chars are bound to the keys and everyone who owns that key can access the account at any time we ban them to prevent item/char loss.
So he will need to purchase a new key from a trusted shop.