PIC5920G is mentioned on this GM 'internal' blog and states they are covered under the Voltec Warranty!!The hybrid powertrain control module 2(HPCM2) and the battery energy control module (BECM) both should have been programmed for this issue and both are covered under the Voltec warranty.
Tell them to look up this bulletin.
#PIC5920G: Vehicle Will Not Charge And Hybrid Loss Of Isolation With DTC P0AA6 And/Or P1FFF P1F0E P0DAA - (Jul 6, 2017)
The labor code above is covered under the Voltec warranty, coverage code: V
Volt and ELR with No Charge Condition Due to Loss of High Voltage Isolation
July 22, 2017
And saved in wayback-machine/internet archive by me just now:
The special labor operation listed in PIC5920G as well as normal programming updates for the HPCM2 and BECM are covered under the Voltec warranty period (8 years/100,000 miles, U.S., or 160,000 kilometers, Canada).