Ask either Robot what button the other one would suggest you should push
to save your friend. Either the Robot is the liar or the truth-teller.
The liar will tell you to push the button that will not save your friend
(since the truth-teller would indicate the other one). The truth-teller
will tell you the button that will not save your friend since the liar
will (falsely) indicate that this button saves the prisoner's life. Thus
in either case you will be steered to the wrong button: you always
choose the other-than-indicated place.