1995 Isuzu Rodeo Question tranny overheating and then smokin

I just sold a vehicle I had owned for nearly four years. I had no problems with the vehicle but I never drove it on the highway or outside city limits. The individual that bought it lived in a rural community, drove it home and then called me stating that the AT warning light had come on and that it was smoking by the time he arrived at home. I have never had this problem before and I am trying to figure out possible causes to decide if I am going to give the guy his money back or make him get the item fixed himself as it may potentially be his error.