I have always been told that work done by friction can, at most, be zero, but never positive. But consider two blocks placed one on top of the other, such that their surfaces in contact are rough. If we give the block on the top a certain horizontal velocity, then in crude words, we can say that friction will try to slow down the block on the top and speed up the other one, thus opposing relative motion. Then in this case, wouldn't the work done by the friction on the block at the bottom, be positive? Please correct me if I have gone wrong.