Defy glass hob cracking

My sister bought a Defy glass top hob from Game with the extended warranty and after a short time the glass cracked from the stove overheating. The plate that overheated was the small plate on the left. When she contacted game to notify them of the glass, then then said that the glass is not covered and gave her the run around and still doing so. The glass cracking is due to the stove itself overheating which is a factory fault and game refuses to acknowledge that fact. After seeing various other reviews on the same product and game choosing not to acknowledge this. This is very disappointing from Game as their policy is not been followed. Very poor from Game. My sister has made numerous calls to Game and Defy and Game is insisting she pays for the glass top, however the problem is not just the glass but the hob itself is overheating, no matter how many times the glass can be repaired, the end result is still gonna be the same.