They should be able to determine if it's a head gasket. Overheating can be caused by many things, but a blown head gasket is one of them, and should be repaired asap if you want to keep using your car. Of course, it a very expensive repair, it may not be worth the $$$. They shoud be able to describe how they determined it was the head gasket (cooling system pressure check, testing for exhaust fumes in the cooling system, coolant in the oil, etc.).
Why do you think they're wrong in their diagnosis?