If he doesn't have a return boarding pass, shouldn't a transit desk be able to issue it?

The flight should arrive and depart from the same gate in the G satellite, which can be accessed from either Terminal 3 or 5. T5 is a remote terminal for check-in; you should try to avoid needing to go there, which would mean exiting to the street and needing to take a landside bus. I'd think the BP can be issued at the gate: the question would be: once you disembark through the segregated halls and get to a transit area, does getting to the departure gates in the satellite require a security check with a BP check?