the Exchange server, or Outlook 2002, is storing the address of the Mac user as being able to understand the proprietary mail attachment format that MS uses.
You can either edit your address book to reflect that this user only takes Plain text or HTML email or do the same with the Exchange server address book. (It's still yours) You will find an option in Exchange to allow Exchange to automatically choose the right format. Uncheck that box. You should NOT send Rich Text as Exchange will create a MIME formated attachment.
Bye the way, this is not just a Mac problem. Exchange can do this to other email clients as well, even on the Windows side.

If nothing works, tell your Mac using friend to download a copy of a free program call TNEF's Enough from here; http://www.joshjacob.com/macdev/tnef/
This will enable him to open the "Winmail.dat" or "application/ms-tnef" attachments.

This has nothing to do with the size of the picture that you send.

Let us know how you/they got on

P