He may have a corrupted mail folder.  If he is lucky, it is the Deleted
Items folder, and not the folder you are trying to delete the messages
from.

For a corrupted Deleted Items folder:
- Close Outlook Express.
- OE5+: Locate the file Deleted Items.dbx in the mail directory and
delete it.  You can use Start, Find (or Search), Files or Folders,
"Deleted Items.dbx" to find it.  On Win2000 you need enable the display
of Hidden or System files in the folder options and XP you need to use
the advanced options to find Hidden or System files.  (You can also
determine the location by looking in OE at Tools, Options, Maintenance,
Store Folder.)
- Open Outlook Express again.  It will create a new empty Deleted Items.

This assumes that there is nothing in the Deleted Items folder that you
need.  If you do, instead of deleting the Deleted Items.dbx file (OE5+),
rename it to something else, such as deleteold.dbx.  Then use the
following program to try and recover messages from it.
http://www.oehelp.com/DBXtract/Default.aspx

If that doesn't help, If the above are OK, then your Sent Items may be
corrupted.
- Close Outlook Express
- Rename the file Sent Items.dbx to Sentold.dbx  You can use Start, Find
(or Search), Files or Folders, "Sent Items.dbx" to find it.   On Win2000
you need enable the display of Hidden or System files in the folder
options and XP you need to use the advanced options to find Hidden or
System files.  (You can also determine the location by looking in OE at
Tools, Options, Maintenance, Store Folder.)
- Open Outlook Express again.
- It will create a new empty Sent Items.
- The following program may be able to recover some of the messages from
the Sentold.dbx file.
 DBXtract - http://www.oehelp.com/DBXtract/Default.aspx
 With a very large DBX file, this can take hours or days.
- You can then open OE and select the Sent Items and drag the recovered
EML files back into it.