Add velocity to that equation. Stopping from 80kph in a 2400lb car will be the same regardless of how much power the engine has.
I've had 5 people in my car and it affects braking a little.
The problem here is he's trying to stop from 120, not 80 - within the same stopping distance. At that point you slow down, or get bigger brakes.
I suggest slowing down, and leaving more time to break. If you get nailed for something at that speed lets just say insurance isn't cheap and trees don't move for cars.