Does the car overheat? Has the dipstick ever flown out? Is there condensation coming from the exhaust? Does it feel as if the car lacks power? If you start the car with the coolant cap off does it bubble? Is there any ticking noises coming from the engine area? Does smoke/steam come out of the exhaust? Does smoke come from the engine area? If the heater is on does it smell like antifreeze? Does it idle ok? Is there any oil/coolant on the spark plugs? Did your mechanic by any chance run a compression test? Does coolant or oil just disappear? Does the car leak any antifreeze or coolant?
BTW These symptoms can be caused by things other than the HG failing such as cracked/warper head or block wich would be even worse but you can tell what exactly is causing it until you tear it apart.