yes because he cant program the motor for it. last tiem I bought the y for two batteries my trek dealer had to call their supplier to even get it authorized to ship.
I am not 100% clear on the scenarios that would really prevent a dual battery to work with the system as is.
In theory, with a battery combiner that outputs the same voltage, the system should have no way to differentiate the amp hour of the new combined battery, and should be able to run from it as is.
Hopefully the firmware update needed in you case has to do with leveraging the dual battery system to do more than what it can do with the single one.
Like manage which battery drains first, or pulling more amps, or adjust the battery level indicator.
In that case it could still work without the firmware, and just use the bigger combined battery the same way as it used the smaller one.
BUT I agree that knowing Bosch closed system mentality, I would not be surprised if they had a way to detect a bigger capacity battery and block the system...
Now, Worst case scenario, if the second battery is the same as the first, he could always just carry the second battery with him on the bike, and just swap them when the first one dies, or create a connector allowing him to disconnect the drained battery and reconnect the fresh one. That should not interfere with the Bosch SW...