I suspect that the particular font cannot be embedded, due to the license that was originally purchased. Some fonts are sold (particularly bundles/families) as "print and preview". This means that they cannot be embedded in documents where the user will be using the font to edit the form. Here are some screen shots of an XFA form which used the "Frutiger LT 45 Com" font, which is "print and preview":
When you look in the Warnings tab, it might tell you that the DIN font does not permit embedding:
When you open the form in Acrobat/Reader on a computer that has the fonts installed, all may appear to be well:
But when you open the form in Acrobat/Reader on a computer that does NOT have the font installed, the DIN font may be substituted:
The DIN font may be a "print and preview only" font, which would mean that it would not be embedded in the form.
The best option is to check the website where the client purchased the font, to check if it is available as "editable and embedded".
If your client really wants to use the font, then an "editable and embedded" font would need to be purchased, if it is available.
I'm experiencing a related font issue in Illustrator: I'm unable to embed fonts in a PDF using Illustrator CS5.5 (Mac OS X 10.7). An Illustrator message says that their is a licensing restriction that prevents embedding the fonts in the PDF. The original ttf and otf fonts were initially purchased for a Windows machine years ago, and many of them came with Illustrator. I've embedded fonts in a PDF using Illustrator (on the Windows side) without a problem for many years.
Is this a font installation issue? Since the "installation" process is different for a mac, is it possible the fonts were not "activated" when they were copied to the library/fonts folder? The fonts do appear in the illustrator file, but not the PDF.
Could this be a Distiller issue? Is Distiller involved when Illustrator saves to a PDF?