Who is at fault? In my opinion 50% the pedestrian, 25% the human driver, 25% technology.
I might question the human occupant component. I hesitate to call them the driver, since they were not actively engaged in the driving process and, as such, would have not been in a position to have executed any actions of a driver within any useful time frame.
My main reasons for this include:
1. As a passive occupant of the vehicle, they would not have engaged in any normal driver activity, such as a constant state of situational awareness expected from a "hands on" driver.
2. As well as their eyes, their hands and feet were not in contact with or positioned for use of the controls. These conditions simply could not occur with a normal "hands on" driver situation.
3. Previous experience of the autonomous vehicle would have trained them in the lack of the need for doing so.
There may be some others, but considering only 1. and 2. above - this is what would have needed to happen.....
a) The "safety driver" would need to have observed the hazard before impact (which I think they may have).
b) They would immediately expect the AV to have responded and they would hesitate.
c) When the AV hadn't responded, they then would have had to remember that they had the ability to take control.
d) They then would have been challenged by the taking the decision to do so.
e) Once they had done that, they would have had to "find" the controls - the steering wheel and brake pedal as a minimum, since their hands (certainly) and feet (possibly) were not in normal driver position.
f) They then would apply whatever action they chose.
For a normal "hands on" driver, only steps a) and f) are involved - the others are not automatic steps and they will take a finite time to be processed.
While a "hands on" driver would have been in a far better position for noticing the pedestrian in the first place, whether they could have been more effective in lessening the severity of the outcome is questionable.