The comments about the controller not being your boss are valid, but completely irrelevant. From the report, the controllers were all that could have saved the pilots from their own error. This isn't a case of controllers initiating a dangerour and erroneous instruction; this is a case of controllers going along with the crew's request to do something that turned out to be deadly. The controllers should have denied the crew's request for a descent to 6000.
Said it before and will say it again the controller is your friend not your boss. Unless there is some other compounding factor (s) surely the pilot/co pilot would have had area maps, the latest satellite information and flight plan for the area as a minimum. Something does not gel here.