Failure description: He charged it up, it sat for about a month, then he rode it in to work. On the way home the display went dark, and shortly after it lost power, dead. He pedaled it home. Charged the battery back up (it was not particularly low on charge in the first place). It remains dead, meaning the display won't turn on and the motor will not run.
Does he happen to know if the battery was running low according to the display on the system before the event, even though it didn't appear to be low per whatever was used to test that at recharge time?
One possibility is that even though the battery appears to be not empty, some cells could have a problem and be either low in capacity or high in resistance, causing unbalance, and if the difference between those cells adn the rest is great enough, the battery's BMS could have turned off the output to prevent damage to the cells or other problems, while still allowing charge. In this event, if the BMS has a balancing function that actually works, leaving it on the charger long enough (hours, days, or weeks) could fix the problem temporarily (a real fix requires replacing the problem cells).
Other possibilities depending on riding conditions, maintenance, etc., are water intrusion into something (buttons, wiring, etc), overload causing overheat of controller and FET failure, overcurrent causing battery BMS FET failure, vibration breaking a connection or wire somewhere (usually in cabling between devices, most often right at a connection or casing exit/entrance point, but sometimes inside a device).
From my research so far, the drive system (hub motor, potted controller, display, crank torque sensor, thumb throttle, and battery) seems to be supplied by one company, Acer MPS (mobile power systems). I could be wrong. But it's one of those closed systems where the battery, display, and controller are all interconnected with some sort of proprietary serial interface. There are two big power terminals on the battery that are not "turning on" meaning the BMS is not supplying voltage to the B+ terminal. I know it's charged though, because there is 42V on one of the smaller pins. Assuming that one is used for the charge indicator since I do see the 42V heading up to the display on two of it's 7 wires.
These types of systems could work in a few ways--they may provide power to the system brains via a smaller wire, and wait for a command or another voltage back from the brain to turn on the main outputs that power the controller FETs, etc.; in this event if the brain wont' turn the system on from this condition, it might be a brain or wiring/connection fault rather than a battery issue.
Without a second battery or system to verify which part is a problem, it's hard to know for sure which one is at fault. Even if you have a power supply that can provide the right voltage to connect in place of the main battery leads (while still leaving all the other battery wires connected), if the battery has a problem with communication to/from the brain, the brain may still not do a normal powerup, so you still don't really know which one is the root cause.
If you have access to the inside of the battery you can verify if the cells are all the same voltage, which would at least eliminate that issue.
If you can get more exact details (from the rider/owner) of the ride leading up to the problem, and of the moments just before it, it might point to more specific scenarios to narrow things down.
Beyond that, unplugging and replugging every connnector, and if that doesn't change anything, then verifying that all connections are good, not just visually but electrically testing from inside one device all the way to the next, would eliminate those issues (but this is time consuming and risks damage to manipulated devices and cables).
What I am really looking for is to see if anyone has experience with a similar proprietary system, maybe a Pedego or other consumer, non-hobbyist system. I am guessing they are all pretty much the same but since I don't know how they work it's hard for me to troubleshoot.
Pedegos are actually pretty generic stuff, the ones I've seen use just main + and - from the battery.
Ones that are proprietary and only work with all-original parts vary a lot, from a simple analog voltage or resistive ID pin (that can sometimes be spoofed) to canbus or modbus or other serial communications, to entirely proprietary setups, most of which can't be bypassed (there are a few that have managed it, like Vandalen's board to go inside Yamaha systems over here:
Yamaha PW series mid-drive motor hacking dongle to use 3rd party batteries Please post your questions about the Yamaha dongle here. See the full article: https://www.avdweb.nl/solar-bike/batteries/battery-hack As with other brands, the Yamaha PW series motor doesn't allow to use any 3e party...
endless-sphere.com
Sudden complete powerlosses during a ride are almost always battery or connection issues, if there were no other events leading up to the problem. Since you weren't riding it, there is a LOT of information you don't have that might lead to different troubleshooting paths if you did. Sometimes the tiniest bit of info that seems unimportant is the key to a solution to some problem. Sometimes it's just useless info that gets in the way of seeing the real issue. :/
Or just no assist, but still has lights, display etc? Or something else? 
