I can't see where the battery is the problem. I'm thinking a computer problem that isn't setting the idle speed right. Or maybe a fuel pump problem that's not pumping enough gas to run it at that speed. How about a clogged fuel filter that's blocking the fuel?
Sometimes when batteries go, they go without warning. If he's able to jump it and keep it running by giving it gas my guess is it's running off the power the alternator is putting out, which is why at idle it dies.