There is no* lounge available before immigration. It is possible for him to wait in the area before immigration, but there are only a couple of cafes and is generally just a transit place as if you are transferring to another flight you pass through the transit station to the departures area (where all the lounges are). You can't get to the departures area without an onward ticket.

There is one Premium Plaza lounge available after you arrive at HKIA, after you have passed through immigration and collected your baggage.
http://www.hongkongairport.com/eng/p...in-lounge.html
It is in the terminal, after you enter the arrivals hall, just near the Airport Express station.
Alternatively, there are plenty of cafes, restaurants and shops also in this area (and levels above).