Is he doing it a regional? If so, chances are that there will be very little time left over after doing systems and ops specs for the APD to ask PTS questions. That said, anything is fair game. But typically, the ops specs, flight planning and systems questions on a typical type ride will more than suffice for the ATP portion of the oral. Areas that I would give special attention to are weather questions with regards to approaches and alternates and fuel planning questions using the flight release. Sounds simple, but you would be amazed how many guys trip over themselves in those areas.