I know I'm jumping in a bit late, but I find it surprising the guard or whatever it's called didn't see the passenger get on.
Yes, "whatever
it's called" (my emphasis). On SWR the person working on the train is known as a guard. They are also a human being, not an inanimate object referred to as 'it'.
The lady in the wheelchair got on the train at Waterloo in rush hour. Even if the guard was on the platform at Waterloo at the time, the sheer volume of people and length of trains using Waterloo means it would be pretty easy to miss a wheelchair boarding. The guard may not have arrived at the platform before boarding commenced for a whole host of reasons.
If a wheelchair user requires assistance to get off the train further down the line (and there are some who don't, either because they are more than capable of doing it themselves either in or out of the chair and others who may have someone travelling with them) , then whoever puts the chair on the train should ensure that the appropriate staff are informed.
In this case, staff were aware but there was a breakdown in the communication chain which meant no-one was immediately available to assist alighting.
Interestingly, the lady in this incident, Sophie Christiansen said in an interview on South Today afterwards that the staff she deals with are great and that her concerns are with the kit and communication system.