Would that person be there if they weren't an instructor? It's their skill and qualification, they're there in order to at the very least take control if the chap in the left seat fouls things up. They are essential to the conduct of the flight.
They're also probably paid so badly already that the flying hours genuinely are part of their remuneration - whatever it says on the contract.
So I'm afraid that I look at it differently. If there's a reason for the instructor's skills to be in the right seat, and that person has the legal and moral right to take over or direct the conduct of the flight - they're captain, and should be logging it as such.
The person in the left hand seat is there because they need that experience, under supervision. So they should be logging it as dual.
That said, the flight should be genuinely needed, not just to build hours for the instructor or on some whim - of course.
G