First of all let me state that I no NOTHING about cars, so please keep that in mind. We're trying to determine if my sister should replace the head gasket or replace the engine all together. She does not have currently the ability to buy a new car. My brother-in-law can probably handle the labor on either option.

 

> We're trying to determine if my sister should replace the head gasket or replace the engine all together.

What symptoms does the car have?

 

First and foremost, how long has the engine sat with a blown head gasket?
Sadly I see many engines that are toast, because the head gasket fails and water gets into cylinder(s) and rusts them while the owner just lets the engine sit, when it just would have been a head surface job and a new gasket.
If this just happened, pull the spark plugs and turn the engine over to pump all the water/coolant out of the cylinders. Then put about 20cc's, about a full cough medicine cup/cap, of auto tranny fluid into each cylinder and turn it over again.
If it's sat several days or more, it's probably going to have rusted cylinder walls, best case it smokes and burns oil, worse case it's seized and it's a boat anchor.

 

Was it driven overheated? Until it stopped running because of overheating? If that happened you should consider the engine to be toast.