The white space warning is part of the Internet Mail, Customize, Heuristics and the user can set this Sensitivity to Custom, this allows for the Customization of Whitespace and other Heuristic settings.
Personally he can’t do anything about what is sent to him but if it is from a single source then they could contact that source and ask them to rectify what is causing the alert.
White space, was commonly used in either the subject or attachment to push parts out of sight, typically with attachments to hide the double file types.
