What should the opponent do when the player knocks over pieces as he presses the clock? Article 7.4.1 of FIDE Laws of Chess states - 


  If a player displaces one or more pieces, he shall re-establish the
  correct position in his own time.


What normally happens when this happens as a player makes his move and presses his clock is that the opponent presses the clock without making a move so that the player puts the knocked over pieces back in place before himself pressing the clock.

But 7.8.1. now says - 


  If the player presses the clock without making a move, it shall be
  considered as an illegal move.


What should the opponent do when the player knocks over pieces as he presses the clock? Particularly at blitz or rapid if he doesn't want to lose because of an illegal move.
Indeed this can be a tricky matter to handle, specially in extreme time trouble.
Pressing the clock quickly again so your opponent adjusts their fallen pieces is fine in general as in most cases the player (or possibly both) will quickly re-adjust everything and nobody will mind, but every now and then you will face opponents trying to call you out on any potentially broken rule in order to claim the game. So to be on the safe side, in such circumstances, you should not press your clock but instead pause the clock and call the arbiter to explain the situation (this is almost always what you should do when your opponent is at fault regarding any rule). The arbiter then relaunches the opponent's clock telling them to re-adjust under their own time (and possibly a warning). This is essentially article 7.3: (this is the exact article, except I've substituted "he" and "his" by "they" and "their" respectively, as to make the statement agnostic to gender, it's shameful how the FIDE laws are written with such disregard!)


  7.3 If a player displaces one or more pieces, they shall re-establish the correct position on their own time. If necessary, either the player
  or their opponent shall stop the clocks and ask for the arbiter’s
  assistance. The arbiter may penalise the player who displaced the
  pieces.


Though I admit this scenario happens very rarely, at least in my experience, as most (courteous) players understand immediately that they have to set their pieces properly once you've quickly pressed your clock again, so there's no conflict. But if you see it's just a messy situation and your opponent couldn't care less about playing properly, then pause and call the arbiter.