You might want to check for the cable pulling or moving in a way that tightens up the brake when you turn.

You seem to have looked at how the pads adjust and how the rotor tracks. Use a flashlight to see if the rotor is running closer to one side in a spot or two, and carefully tweak it.

Check the caliper to make sure it is pretty well lined up with the disc.

You could also try backing out both pads until you get no rubbing at all. You might have a fixed pad that moves in clicks or detents. Move it in until it just begins to rub. Then back it out until it just stops. Then back it way out while counting the clicks or turns. With it backed off, set your moving pad until it just barely doesn't rub. Then return the fixed, countable pad, back to it's no rub position.

You'll might also want to squeze the brake regularly as you make all your adjustments, as it might affect the final pad position after an adjustment.

It could also be that the pads are held a little loosely, and can move a bit. As long as they seem to be secure as designed, this may not be a problem.

You might find that you have to compromise between a little rub and nice tight brakes. I wouldn't get to worried about a little rubbing, expecially on cheaper brakes. You friend probably won't even notice it when he rides.