I am just posting this as a possible solution for folks. It worked for us.
* User was on OS X
* Our solution (hosted) uses the Calibri font (a Microsoft font)
* User did have Calibri a font installed on their system; this font was available in other applications
* This one user was seeing the layout rendered in Arial and not in Calibri
* Calibri was not visible as an option in the 'Font' menu of FileMaker
* Run the 'Validate Font' function in the Font Book application (can be accessed by right-clicking on the Font name)
This resulted immediately in the layout rendering correctly with Calibri and the font showing up in the FM menu. It's a very quick process. The process didn't report any errors, oddly.