go to autozone or somewhere that has a scan tool. Pull the codes and that will give you some idea what you are looking at....I had the same problem on my 02 GT auto, codes indicated that there was not a drop in rpm's from 2nd to 3rd....I flushed the tranny, changed the filter, filled, and reset computer....no o/d light since. Also, has he done a tune and changed the tranny shift pressure? If so, he might want to put shift pressure back to factory and see if light comes back. Sometimes it turns out you have a shift solenoid going bad.