Generally speaking cars are not designed with the contingency of a passenger needing to stop the vehicle in case of an emergency. Depending on the vehicle, the same issues would apply to a manual car as an automatic one, in that the passenger is unlikely to be able to reach the pedals and so cannot operate the footbrake. In an emergency where the driver became incapacitated, we would suggest moving the vehicle's transmission into neutral and using the handbrake to slow the vehicle gradually while steering if possible, but this would only be as a last resort.