If the 12V battery connection comes lose, the DC-DC can still provide power to the vehicle. If any component in the HV system fails, the 12V battery still keeps the electronics powered on. If she was able to set the brake, that means there was power to the brake switch, which goes to the body control module IIRC, and power to the brake actuators. Did she try using the hazard lights, or were the headlights on to indicate if there was power available to other components?
Going "dead" and losing 12V power to all devices simultaneously, but then having them come back with a power cycle, is hard for me to imagine. If there was significantly greater voltage than 12V on the electronics, they may shut off (?), but that's a stretch for sure. If there's significantly lower voltage than 12V on the electronics, they would shut off eventually, but then a simple power cycle wouldn't solve that problem.
If this happened to me, I'd absolutely take it to the dealer. The description is hard to imagine, especially without any codes, but the dealer would be able to check all the obvious stuff like connections, voltage output from the 12V battery and DC-DC, etc. to put on record that you've tried to address this issue.
Bryce