Google is not responsible in any way. Its not "not as clear cut." The only unsure thing is easilly remedied if they examine the facts of the incident.
If anything the City/state/province is at fault for:
1. Not putting up Night reflective signs AND barriers if a road goes from pavement/gravel to OFF A CLIFF.
2. Not giving good data to Google of which Google would have to later fix by hand. Considering the sheer amount of data they get in, this is not an easy or immediate task. Some areas might not have good info to give to Google, which is why this can sometimes happen.
The driver is mostly at fault if:
1. He ignored any road signs, went around barriers, or went off road.
2. When visibility went to near zero, not having the passenger walk ahead and sight for him (Night vision can be better outside of a car especially if you can see from a higher angle than sitting down in a car).
3. Not use high visibility features of the car: Foglamps, or Highbeams. Foglamps are not common in the US, but I am not sure about Spain.