MPH at which battery goes off & gas-only kicks in

Your friend should look at the Energy display on his car. Electric assistance works at any speed on a Prius. Look for yellow arrows from the motor to the wheels on the display.

The 'normal' running mode for the car, at a steady speed on level ground, is the engine running with power going to recharge the battery. However, the system will periodically send some energy from the battery to the wheels, to prevent overcharging. If the battery gets into the green part of the display (seven or eight bars on the eight-bar charge indicator) the car will use more electric assist to discharge back down to six bars.

The Prius mostly achieves its fuel economy through the Atkinson-cycle engine, which uses more of the energy available in the fuel (up to 36%, compared to a regular Otto-cycle petrol engine which gets around 20%). The battery has about enough capacity for a couple of miles of driving at 30mph.

The engine shuts down and the car uses battery only ('golf cart mode') whenever the required total power to the wheels is less than the engine can supply efficiently, the battery's state of charge is sufficient (more than two bars), the engine is hot enough, and the speed is below 42mph. Above 42mph, the engine has to spin but is not necessarily providing power (this is to prevent one of the electric motors from over-revving).

 

As an additional point, it's important to realize that power management in the Prius is complex and sophisticated. It changes by the sub-second, influenced by many variables. There is no magic "gas only" or "electric only" speed.

Tom

 

FYI,
Following chart is came from 2G Prius to understand how THS works.
The site URL is...
TOYOTA: Company > Technology
It's written for the 2G Prius, but it's worth to read.
TOYOTA: Company > Technology

Ken@Japan