I'm afraid we can't change usernames on existing accounts, but I believe our server master (Razmirz) is looking into this error as we speak.
I'll let him be aware of this, since I think this might be related to what he's working on, but just have your friend wait until registration reopens the next time (keep checking into the site every few hours), and hopefully the issue will be resolved by then.
(If not, PM me and I can continue to look into this issue for you.)