It stopped working for him one day and it had been there for around 3 months.
If the Powerbook was running and suddenly shut down, and it hasn't run since, it undoubtedly had a hardware failure of some significant kind. In that case, there's nothing surprising about its failure to start now. It's a good guess that the backup battery (that array of four disc cells you mentioned) was drained when you received the PB. But if you've followed the usual suggestions to charge both batteries, and the PB still won't start, even on AC power with an adapter that's known to be good, it needs expert testing and repair.
If you don't have an AC adapter that you
know is good, take the PB to an Apple dealership and ask to use a demo adapter on the display floor for a few minutes to test whether the results are the same as with your own adapter when you connect it to the PB.
I still can't imagine what you mean by "the main battery starts to flash". What is
really flashing? The sleep light? The AC adapter plug light? The Caps Lock or Num Lock light? What?
My guess is that you've got a failed AC adapter, logic board or PMU, but it's also possible that the main battery's connection with the logic board is loose, or the DC-in port's solder joints to the logic board are cracked, or the port is otherwise damaged. I'm sure there are other possibilities I haven't thought of. Unfortunately, diagnosing these power-related problems (if you can't find a loose connection somewhere) is usually a matter of replacing parts until you figure out which one was faulty.