Is there not a process for the Guard to inform the driver passenger assistance is required at a DOO station?
This is done by platform staff in the London area (they should notify the guard/driver of where the person is getting off). In the guarded area the guard simply won’t dispatch until they’ve completed assistance.
Short answer; it isn’t, the metro locations are monitors, bat and flag or CD/RA dispatch to driver, with no guard involvement, and the guard
should always join the train at its origin in London. I’m not sure whether platform staff or driver is responsible for checking the guard is present.
If the train was dispatched without the guard due to an error by platform staff/driver you’d expect said guard to call control immediately to inform them, so it seems possible that in this case the guard was unavailable but the message somehow wasn’t passed on, and that this also wasn’t noticed by the driver or platform staff.