2002 Chevy Venture Slipping in overdrive

OK this is a florist delivery van that's use were my daughter works. They do some maintenance but have never changed the transmission fluid or filter. Now the real problem, the are telling her she caused the transmission to slip by the way she drives. I taught her around town and going down steep hills don't put it in overdrive.(Towing also) On the highway or over 40mph put it in overdrive. She does have a habit to drop it down out of overdrive when she is coming into a slower speed zone like 40mph or slower. Would this cause the slipping in the transmission? They are giving a hard time about this. I have never had a problem and have done this for over 30 years.