Batteries and Bus Bars

What do you mean, by his battery "went out"? What codes did you get from the battery ecu? Unless there is corrosion damage to the battery voltage sense harness or the orange connector at the battery ecu, then you likely have a faulty module. You can do a load test on each module (record the starting voltage, apply a load such as a H4 halogen bulb- or two, for two minutes exactly, then record the end voltage just before removing the load). If a module is really bad it might show up, but 5-10 amps from a headlight bulb or two is much less than what the car normally uses.

A better method is to use a scantool (such as Techstream) or a OBD2 bluetooth adapter with Dr Prius or Hybrid Assistant app to monitor all the battery block voltages while driving the car. High amperage charging and discharging from aggressive driving will "flush out" weak modules. Up to 160A on heavy accel and 90-100A regen.

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Since it seems you've already disassembled the battery, you should also test for ground faults. There is a small threaded steel insert that is pressed into the bottom of each module that allows the module to be fastened to the baseplate. Often, the plastic module will develop a crack/defect that will allow internal electrolyte to make contact with this insert inside the module where it can't be seen. When this happens, it will trigger a P0AA6 code and if bad enough, will cause the main relays to open. If you place one lead of a voltmeter in the insert and the other lead on each of the module terminals, a good module should effectively read 0 volts. Sometimes it may read a higher voltage and rapidly decay down to 0v. A module with this internal defect will read a steady DC voltage, meaning the terminal and the insert have continuity with each other via the electrolyte. That module, even though it may be powerful, is still a bad module.