Ok. I'll call him ASAP tomorrow and beg him to flush the transmission fluid. His dad will probably help him. The car does drive very smoothly now, though.

The problem with other fluids is that they are too slippery, and the trans controller had trouble with applying the clutches. It reads too much slip, so it applies the solenoid a little more, and the clutch abruptly catches. The controller reads the grabbing clutch, and reduces pressure, the clutches start slipping, and the cycle repeats. 24 times per second.
It eats clutch material away like you wouldn't believe.

The problem is that it shifts really smoothly while it'* doing this, right until the point that it burns enough material off to start slipping, and by then the tranny is junk. The case, electronics, and gears might be reusable, but the rest of the internals are toast.

Yes, that is disgusting! Thanks for the explanation.