Sure enough, somehow the font option for reading and composing plain text emails had been set to a white font. I quickly determined that it was only plain text emails that were affected (rather than those with HTML or Rich Text formatting), which led me to investigate the font options. He knew there was text there, because he could highlight the invisible text and copy & paste it into Word. Shortly after upgrading to Outlook 2013 from Outlook 2010, one of my users complained that some of his emails were showing up with invisible text in the message body.
