Agreed, it's going to be very expensive to fix if there's more than damage to the door. Fox bodies do very well in slow-speed collisions...up to a point, and then it's not even worth fixing. The door itself is minor to replace, as previously stated; but as
Unit5302 points out, anything more would probably cost a LOT, mebbe even more than the car is worth, sorry to say. I would get an estimate and go from there. Unless the rest of the car is spotless and it has low miles, I'd say total it and let it go.
THEN get the 89 Corolla for him.
Oh, BTW, if you can take some good pictures of the damaged area, please post them here and we'll give you further remarks about it. A picture is worth a thousand words, ya know.