Is that even necessary? I always thought all you needed was an F boarding pass or DM/OW Emerald status and continuing OW boarding pass to enter any OW F lounge. As I understand it, simply his boarding pass should suffice as permission to re-enter any CX lounge.

From the CX Website:
The Pier - First Class Lounge (Airside)
Admittance
First Class Passengers
(Passengers on either Cathay Pacific or Dragonair flights)
Diamond Card members
oneworld Emerald members

It is the same for the Wing. It doesn't really say anything about about re-entry stamps. And I've used both my Y (with status designation) and sometimes J boarding passes to re-enter lounges before.