Was I stupid to have this Repair Shop fix what caused my engine to Overheat?

My car is old but I have always had the oil changed and fluids topped off. The engine overheated last week. I brought it to a cheaper repair shop to see if they could tell me if they could fix it. I have used them before for mufflers and brakes and they do good work on those things. They found a radiator leak and other things but I drove it the next day and it overheated again. They said it was due to a popped head gasket due to driving it without anti-freeze. I know the engine could be ruined but I wish I hadn't paid $500 to have them fix the radiator, do a tune up and so on . . . just to have a dead car. He won't give me my money back because he said that they didn't see the problem with the anti-freeze/head gasket at the time of the repair. Comments? Don't really want to sue. Will have to eat the repair expense and get another car . . . just looking for someone experienced with auto repairs to give me insight - e.g. how stupid was I?