My guess is that he has not got Word installed, or if he has it may be the wrong version.
In previous versions of Word the file extension is DOC. If he has not got Word then Vista will not know what to do with a DOC file.
In Office 2007 the file extension is XDOC, and if he has not got Word 2007 then his Vista will not know what do to with an XDOC file.
Can you confirm that he has got Word installed, and what version.
Can you also check which version of Word you have.
If he has not got Word then you can convert YOUR word file to a PDF file very easily with some free utilities on the web.