Yes? So they identify themselves as a commuter and security grants them access as per existing regulations at all airports.
Do they also identify themselves as not being on duty?
Like 4468 previously said, if they identify themselves as commuting but OFF duty then they should be redirected to passenger search. Regardless of what security tell them or allow them to do, it is their pass that is on the line.
If they identify themselves as commuting but ON duty then by all means carry on, that is what the whole point of having airside passes and staff channels is about! Not to access airside off duty, therefore with no legitimate reason to be present airside.
To present it another way, how would it be different for example from the son of an FR pilot who happens to work at the airport in another capacity (cleaner, but holds an airside ID) who wants to fly with his father who is flying out of the airport. The son/daughter is not on duty, they are on a day off, but they go through staff security using their airside ID and then proceed to the gate where their father is the pilot of the flight and they gain access to the aircraft and depart with their father out of the airport.
Is that or isn't that a breach of DfT regulations? If it is, then it is also a breach regardless of who does it, be they aircrew, ground crew, shop staff or cleaners.