He can't get in, or he gets in and the emails are already gone?
If they're gone, you can, in Microsoft Outlook account settings check a box that says 'Leave a copy of messages on the server' (Tools - email accounts -view or change existing accounts - double click on the account - 'More settings' - 'Advanced')
If it's that he can't get in at all...that I don't know how to solve
good luck though!