The raw source (as in the message headers and all) would be most useful. That almost looks corrupt or perhaps an add-on gone wrong (do you have any that manipulate emails?).
I'm not familiar with the application he's using (not sure if that's Outlook these days, haven't used it in a number of years), but I'd guess so. If it's empty, it might just indicate some corrupted mail delivery. I don't think XF could generate that, so probably just keep an eye out for it again.