Honestly reading this it sounds like they have likely logged into the wrong account when they were at your house.
If your friend gets in touch with Support using the link Ananda gave above, the team there should be able to help them locate the account and get them back into the old characters hopefully.
Any info they can remember about the old account will make that easier, old emails, character names and realms, cdkeys if your friend still has them etc, the more they can include in the ticket the better 
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