Knocking the car out of gear and easing the handbrake on would also work
given the clutch was down the gear stick was irrelevant. In my car the handbrake lever is to the right of the driver, on Merc’s it’s ont he floor by the drivers feet.
In hindsight, saying ‘check you have your foot on the right pedal’ might have helped more than just shouting brake slightly more hysterically, though.