To be fair I’m not sure you can win that fight. There’s no way you can prove that you yourself didnt smoke the head gasket. They have no way of knowing how long or short you drove it while overheating. While you probably didnt cause it they have no way to know. Car came in with a bad pump and had already overheated before arriving. If it were my shop I wouldve had them pressure test the system before letting it go out on the road for testing to cover our asses but I’m not sure that will matter in the end.