This old model had poor protection for the underlying logic board if liquid got into the keyboard. I once opened one up and found blue-green crystals all over the logic board, crystals that gave off a feral smell. Turned out the person's daughter (the user) had a pet ferret who liked to run across the keyboard... and she did not realize that ***ferrets pee as they run***. It had sprinkled her keyboard over and over until the unit shorted out. That was just ... nasty.
Pull the RAM chips, and check for corrosion on the connections – a sure sign of further liquid damage. Those old white MacBooks were a giant pain to work on for anything more than a drive swap, battery replacement, or RAM upgrade, and are worth less than a tank of gas these days. ***My Opinion:*** The repair isn't worth it. Pull the drive, save the data, and advise them to get something newer. Doesn't have to be new – not all of us have that money – but something within the last 6-8 years. Perhaps you even have such a used unit already that you could sell them?