Was on teamspeak with him last night, he deleted the cookies properly. To no avail. Switching browers did not work either.
Only possible solution I could give him was to change his hero to another character and back to his current main in hope that this will work.
Or change the name back to the old name, then change character, rename the current main and change back to it.
And thinking about it, maybe it would be worth trying to log out of the website and back in?