I'm sure you are not allowed to. I'm also sure in over 500 segments, I've never seen a member of the flight crew check ID.

I would presume most human service directors would be fine with a straight up swap of seats of people who look the same, less a generation of weathering, but also expect little of others.

You could probably make a point of escorting him down during Zone 1, and that might mean someone has to think for a second about who is going where, but turning left to the pods or right to PY would happen after anyone obligated to check and ID has checked ID.

I'm not telling you to either lie, or to roll the dice. But all situations where you are under the gun, I would fully expect you to be able to mumble through to your desired outcome.