Perhaps you misunderstand. He is already stopping the engine by hand with the level at the injection pump.
But he wants to stop the engine with the stop button at the engine panel using the stop solenoid as normal.
Regarding the wire diagrams it might be he has first switched off the start switch and after that the stop button has no current anymore.
So first stop the engine with the stop button and after that switch off the start switch (using the key).
Perhaps you will want to take this correct procedure into your Amel documentations.
Amel Sharki #60
Currently Kalamata, Greece