Cheve 1500 slipping out of OD?

I have a friend that has a Cheve Silverado with about 100k mi. It has the tow/haul package..about an 05 model. He will be driving down the road and with no warning, the OD will kick out. It will be under no load or acceleration. He bought the fluid and filter to service the transmission (never has been) but a bunch of guys told him that changing the transmission fluid now could tear it up??? I know the concept of the transmission and torque converter....I drive a F350 and learned a LOT about transmissions with it. So any ideas about the potential problem would be helpful. I think he should start with the fluid/filter. His fluid in it now does not smell burnt.