Without any boarding pass (for the day he arrives), the OP will likely have difficulty getting behind security at Terminal B near the Admirals Club. He should call Admirals ahead and ask though, and he might ask again at security upon arrival. For the record though, I used a VX boarding pass (terminal 4) to access the Admirals Club at Terminal 8 this week at JFK. I gained access at the Admirals Club using my AAdvantage credit card with Admirals Club privileges. Security at terminal 8 didn't ask why I wanted to enter term 8. If i had a day pass, I think the Club would have let me in.

Maybe a day pass to one of the airport hotels is a better option.