We do observer training here at merseyside. I actually teach the nav package to some courses and certainly fly and assess a large number of course training sorties. I taught low level nav in the military as well.
THIS IS NOT A SAFETY PROBLEM.
The pilot is THE flight crew member........ full stop. He is ASSISTED by the observers who should assume the tactical navigation for the sortie and police job at hand. If he busts controlled airspace or danger areas, guess who is at fault?? Do not under any circumstances blame student B for the safety of the aircraft.
If a student fails the course for navigation, it is usually because they cannot control the tactical navigation aspects that are required for the job. As Farmer 1 states, some people take extra time (often ending up as brilliant observers) and some will just never get it in a million years. This is important to differentiate, the bad observers lose the success of the police incident at hand NOT the safety of the aircraft.
I hate to say this, but anyone who disagrees is operating the aircraft in an incorrect manner IMHO.