If his pads still have plenty of life left there is no reason put brand new pads on.
Kernal I think you should be alright just putting them back in on either side, as long as the surfaces look to be in good shape.
Kernal, I would think you would be able to visually look at them and determine which direction they are installed, based on ware patterns? I could be off on this, but it seems to make sense.