if those mails were not deleted by mistake i am sure those mails must have been archived. go to Outlook 2007 tools – >Other, there you will see an Auto archive option. from there you can see the archive file location and mount the archive file to Outlook 2007. you may able to find your mails
Since you say this is only happening to ONE user, you can rule the Exchange server out as the source of the problem. Did the user bring this to your attention? Have you checked their deleted items and “recover deleted items” folder to confirm the user did not delete them himself? If the date of July 13th is the date at and before the sent items have disappeared it is unlikely they were archived, however check the archive settings on that specific folder to see what they are. The user could have changed the setting and not realized what they had set it to. Right-click the sent items, go to properties and then check the autoarchive tab. Also, has the user given permission to his mailbox and all folders to someone else? If someone else has access to this mailbox and deletes the mail in any folder it will go into the OTHER user’s deleted items, not the mailbox owner’s deleted items.