Assuming he uses Outlook Express:
Tools/Options
Go to the Security tab
Uncheck the box "Do not allow attachments to be saved or opened that could potentially be a virus"
Ignoring the appalling grammar here, the key word is POTENTIALLY. As far as MS is concerned that means just about any attachment, so if he has this box checked he will get a warning telling him that he was prevented from opening a file that is potentially a virus. It doesn't mean it IS a virus.
It's a bit like telling someone "Don't answer the door; the caller may potentially be a serial killer".