1990 Mercury Sable Heater core replacement.

My nephew, found a 1990 mercury sable, to get him back and forth to college, but it needs a new heater core, I know a friend mine had his replace and it cost about 800. Well that is more than the cost of the car. My father-in-law and I were wonder if we could do? My father-in-law he tried to do one on a older car and worked on it for 2 day before giving up, but thought that since was a newer car than the one did changes, might have made them easier to do. Do anyone know. If this is to big/hard a job for us to tackle?