Hahaha uh oh... Someone forgot to restore a shelved polygon before going to print! If he was competent, he would have run a DRC prior to exporting the Gerbers. This is standard procedure, especially for a board this complex. It would have warned him about the missing ground plane (would be detected as an un-routed net or net antennae)
...UNLESS, all he has are the Gerbers, and not the source files? Or maybe he switched the DRC warnings off entirely?