You probably have internal engine damage. When a car overheats the aluminum cylinder head tends to warp causing the valves to get stuck and/or damaged because they get hit by the pistons. Because later model cars have smarter computers that are more forgiving and are able to make the car perform well when something is really wrong.
I think that in your case, when the car ran hot, your girlfriend had no idea nothing was wrong so she kept driving around on her merry way. She was probably able to drive it for quite some time before it started to lose power and eventually die.