From the RAIB the relevant timings are:
20:22 - train departs West Worthing station
20:25 - train arrives in siding
20:25:25-
20:26:04 - driver walks through train
20:41 - signaller clears route from siding, presumably at the booked time
20:49 - guard is contacted
20:53:50 - driver of second train reports driving cab empty, discovers driver during the call.
It seems therefore that there was approximately 19 minutes between the guard having ceased their duties and the earliest possible moment for them to realise something was wrong, in reality that's going to be more like 20-23 minutes as they won't know exactly when the signal was cleared, and the first thought would not be that something is wrong with the driver - particularly as they had at most a few seconds of interaction. So it would be at best a growing realisation, and this all assumes the guard wasn't preoccupied with something else (If it is acceptable for the driver to be using their personal phone during this time then it must also be acceptable for the guard). This leaves, at absolute most, 7 minutes between it being possible to know something is wrong and being contacted, in reality almost certainly less than 5.
It seems likely that when they were contacted the guard was at the opposite end of the train to the driver, it would take at least 1-2 minutes from contact from control being initiated through the guard reacting, answering, the information being given, coming to an understanding, signing off, safely stowing anything they had in their hands and needed to take with them (radio, personal phone, ticket machine, etc) and starting to walk through the train. It took the driver 1 minute to walk cab to cab, the guard would be slower as they will be looking for the driver on the way. This means they could not have reached the driving cab until at least 3-4 minutes after being contacted - i.e. approximately the same time as the other driver was making their report. At that time the guard would not have been able to report any more than the guard apparently not being on the train, and it is most likely that this report would be to their control not the signaller, who they would then need to contact separately if they needed to investigate further, by which point the driver has been found.
I don't really see what else the guard could have been expected to do.