Older displays had the LCD panel behind a glass cover glass. Today all of the Retina displays use a laminated cover glass so the LCD screen is bonded to the cover glass. What you are likely seeing is the LCD glass due to the processing of it tends to get brittle so the break can take place within the LCD part and not the cover glass. Often I see the back side glass breaking before the front.
In any case its time to replace the screen [product|IF188-144] and here's the guide [guide|142674]
The owner of the system will need to be a bit more careful as sitting something heavy on the system or a good bang is all it takes to kill the display.