Presumably she doesn't run an exchange server so to be clear which email service is she using? I had Hotmail that was turned to Windows Live and all my messages disappeared one day.
Most people use Outlook as the email client but the messages are kept (and lost) elsewhere.
Edit - on the same theme presumably if she is taking the messages from elsewhere she has it set to delete messages from the actual inbox?
Otherwise search in outlook for the option to recover deleted messages.