DDR Extreme == Sys573 Digital. It's the newest mix you can upgrade a pre-SN machine to without replacing the whole computer, so it's overwhelmingly common. I thought they used a Flash ROM on the security cart to store scores and settings... guess I was wrong.
So, it boots and plays despite the error? That's certainly a good sign.
I'd just pull the factory battery, and add a battery in the cabinet with the same voltage and run wires. Does the customer happen to have any working 573 games? (Not necessarily DDR; cross-reference his gameroom with
System16 (also
here and
here) to find a suitable reference machine to get the intended batttery voltage.)