The solution depends on the real nature of the problem, which you forgot to describe. I doubt if it is any mail client compatibility issue, however.
It sounds like one of 2 things is happening.
The simplest thing is that the recipient might not be able to open the file. If the recipient has Word 95, and your guy is sending a Word 97 or higher file, then the recipient is going to have problems. Specifically, their Word is going to complain and be unable to open the file.
This fix for this is: have your guy "Save As...", either Word 95, or RTF. Or you can encourage the recipient to upgrade - good luck if they haven't done this by now.
Another area that may be generating problems is that the recipients' email systems are stripping the attachment. Some places do this. If this is the case, the recipient will simply not have an attachment. You need to check if this is the situation.
The fix is for the recipient to change their email system, if possible.
Another possibility is that the recipients' email systems are modifying the attachments. Like "StillAtIt" mentioned, maybe an AntiVirus tool is "cleaning" the virus. This could be either because of a badly configured email/AV system at their end, or perhaps because your guy has an actual virus that is getting in the email/attachment.
So do you have any details about the error at the other end? If they open up the email they do receive from your guy, and select Properties from the File menu, they will see a dialogue. Depending on the email client, click the "Details" or "Internet" tab to reveal the header. Have them select everything in that view, and paste it as raw text into an email back to you. It will show you if there are any problems with the email transport side of things.
