If your client is receiving the PDF as a Proof document and opens it in Acrobat Reader, there should be no problem if "any" fonts are embedded. If however they choose to use a different Program to open the PDF, yes, there will be a hassle, if they don't have the fonts installed.

Why not lock the document to stop them from using any other Program to avoid any font substitution problems? (Just activate the permissions password).

Just a thought,

Cheers - George