There is a Purdey and a Greener at the upcoming Julia Auction. The Purdey has a high estimate but the Greener isn't too bad. Probably some US stuff in the auction also but I didn't take notice.
There are always quite a few 10 gauge Parkers for sale on the net at any given time. Does he want something new or vintage? Steel or damascus?
I picked up a Westley Richards 10 gauge last year on gunbroker for a song.
Found quite a few that didn't interest me when I was looking for mine. If you can give me a better idea of what he wants I may be able to steer him in the right direction.

Josh