calista, don't kid yourself, legally it is theft.

http://en.wikipedia.org/wiki/Theft

Especially take note of the parts about actus rea and mens rea.

I'm also pretty sure that based on your post you could be compelled to identify him.

Given that as part of the cleanup of the MTGox situation, people will be watching this forum, it's probably a good idea for your friend to give it back before they or the bean counting catches up with him(and you).

Even if you hadn't posted here, there's a pretty good chance they'd find the error as they continue to make passes over 'the books' as part of reconciling all the accounts, transactions, etc.

Simply ditching an account and making a new one wouldn't matter unless he has been hiding his IP and such. They'll have logs and so will his ISP.