The car brakes were likely not faulty. That’s still a relatively new Mercedes, and most modern cars (even Kias) generally have working systems. Especially the ‘important’ systems, like brakes.
Most likely is she pressed on the gas pedal rather than the brakes. I’ve seen enough people do that exact ‘move’ at local 711s to know it’s almost always the core reason.
Remember the Audi “unintended acceleration” scare in the 80s? Same thing, people were hitting the gas instead of the brake and then panicking and pushing the pedal even harder.
I’ve never had brakes fail outright, though I have certainly had old VW drum brakes fade to practically nothing.