My nephew in Florida changed his battery in his 1980 f150 302 and connected the cables to the wrong post. what damaged could have happened by going this. he's replaced the solenoid, but still has no power anywhere, no lights horn will not start.

 

There may be a fuseable link on the positive battery cable that has fried.
Google up the truck and engine searching for fuseable link locations

 

could have fried the ecu. In those days there weren't that many other modules

 

My daughter got a new battery installed at GoofyLube and it was installed backwards. Multiple failures from alternator to ecu to fuses! Major fu. She owns part of GL now. Ha. Jk. But, your nephew has some weeks of finding problems and replacing parts.