Seems possible to me that the elbow was cracked with a solid sewer snake that couldn't make the bend because of the concrete. Then the jetter could have finished the job. But at any rate, if I knew there was concrete in the pipe like that, I think I'd recommend digging and repairing. It's the owner's problem - he's the one with the concrete in the pipe. Heck, they MAKE pipes out of concrete - how can you be expected to clean a pipe full of it?