Antifreeze problem

My neighbor has a 91 Camry with a 3S-FE motor. He asked me to look at the antifreeze because he said it was brown. I drained the radiator and sure enough, it looked like chocolate milk. I filled the radiator full of water and drove it around for 10 minutes with the heat on and drained it again. Once again, chocolate milk. I filled it up again and drove it again, etc. I must have put 10 gallons of water into it by now and it still looks brown, although I think it's getting a little lighter. The car never overheats and the temp gauge is always at the same position, just about right in the middle. The car does have 230,000 miles on it, but I know he has had the radiator flushed in the past.

What could be causing this? Can there be that much gunk in the radiator? What should I do? Any help greatly appreciated.

The inside of the radiator is probably rusted or rusting out. The best way to fix this would be to replace the radiator.