The e.c.u. jamming it into 3rd is a definite sign of the transmission being in Limp-In Mode.
Sometimes the outside electrical connectors require a cleaning and careful tweaking of a pin or two, as they can cause the Limp-In issue through improper grounding. This sends the wrong info to the e.c.u. and forces the transmission to use third.
What's odd is there is no Check-Engine light. There MAY be a code resident if he uses the key on, off, on, off ON procedure (?)
I've yet to see a fluid/filter change and/or band adjustment "repair" issues like he is having (particularly slipping). Once slipping starts it takes no time at all before friction materials clog those tiny Governor screens, line pressure is lost and it's game over.