Address Book uses the Helvetica font included with Mac OS X. Other kinds of "Helvetica" fonts might be installed, but Address Book requires the Apple font.
Use Font Book to
enable Helvetica in the All Fonts font collection.
Tip: If you use third-party font management software, temporarily disable it.
This document will be updated as more information becomes available.
Additional information
The Apple version of Helvetica is located in this path: System/Library/Fonts/Helvetica.dfont. It may appear there even when it's disabled. Using Font Book's Resolve Duplicates feature may inadvertently cause this issue, depending on what other fonts you have installed.
The Console utility displays this message in the console.log when the issue occurs:
Address Book[3225] Exception raised during posting of notification.
Ignored. exception: *** -[ABTextView setFont:]: nil NSFont given.
The number in brackets may differ.