What version of Word is he using?
Word v.X file format is exactly the same as MacWord98/01 and
WinWord97/00/02/03 formats.
If your correspondent uses Word95, then I'd suggest saving as RTF -
which can be read by Word95, and will preserve most if not all of the
formatting.
If your correspondent uses Word97/00/02/03, the problem may lie in a
couple areas. One is your encoding method when you send a file. Take a
look here:
http://www.entourage.mvps.org/faq/faqs.html#anchor-17
Sometimes, even with proper encoding, mail servers munge your data. When
that happens I use Aladdin's DropZip to compress the file, which seems
to fare better than the raw Word doc.
Also, make sure that your files have extensions (i.e, "filename.doc",
not just "filename") - it's not strictly necessary, but many Windows
users don't know how to associate files with applications.