Check sender's email client (View all Header settings). There are sometimes issues with Thunderbird clients. Check sender's encoding settings as well (open GM email in browser and select source code view). Some clients using exotic encodings.
Check CEO's MSHTML.DLL settings in Internet Explorer. Be sure he is using UTF-8.
Try to set ISO-8859-15 for this user.
!!This settings will only work with new messages, and will not repair any existing messages.!!