Help With Powerbook Diagnosis :-(

My friend spilt wine on his g4 powerbook a month or two back. He dried it out for a few days after the spillage, then turned it on after transferring files off the hard drive, booted up fine and worked no probs. It continued working for about 3 weeks, then froze up. On trying to boot it up again the hard drive spun but the screen stayed black, this persisted for about ten minutes and wouldn't shut down by holding down the power button, then what sounded like a large fan located at the back of the keyboard near the screen came on and the poweerbook shutdown. Since then its only booted up intermittently and has a tendency to freeze. He paid to have it checked out by an authorised Mac repair company and was told the logic board was faulty/damaged and it would cost approx £700 to replace! How they determined this is unsure because they didn't even open the laptop up to take a look inside . He took it home and managed to get it on again, he did an extended hardware test, verified and repaired permissions and everything checked out ok.
I know it's not a lot of info to work from but does anyone have an idea of what might be the problem, can it be fixed? Might it be a problem with the power input, a short circuit or something? Or is it indeed the logic board and if so what does that mean and how is it determined?
Any help or advice would be greatly appreciated.