If you can provoke RB2 to send something, e.g. just reply to ARP request, then you can use Tools->Packet Sniffer to capture the traffic and get MAC address from there. If not, you might use the same and hope that RB2 sends something by itself.
When it is already dead, start a trace on the port of the first router and have the second router rebooted by the client.
You should see some activity when it boots.
You haven’t written anything regarding the physical accessibility of the devices, maybe the customer could read the MAC from the device’s sticker before you jump into the car?
The MAC addresses form up a contiguous block, the sticker normally contains the address of ether1 and wlan1.