When Opponent Moves But Doesn't Press Clock

This happens for a few split seconds every move. I guess you sit back and relax and watch him run out of time.

As a courtesy, you might point a finger at the clock, indicating that it's still your opponents move.
Or sit back and wait till he or she runs out of the time and claim a victory.

I make my move, then sit there, and don't press the clock. (which is illegal) XD

I beat the child to within an inch of death for not knowing every minute detail of this silly game.
Seriously, this has never happened to me, but I've seen many children do this. The most often response is the make the move, realize that the opponent didn't press the clock, press it for him, then press your own clock. Those who notice before making a move don't move any piece. Only once have I seen the child giggle giving the opponent a clue that something was amiss.

it is a part of being an ethical player to point a finger at the clock when your opponent forgets to press it.

I don't get the "ethics" argument. Unless the opponent is completely new to timed games, it's 100% their fault if they forget to press the clock. Might as well enjoy the free time and let them learn a lesson on vigilance.

Ethics is basic to otb chess...if you don't "get it" you obviously never learned playing otb.
Ethics is another term for "good sportsmanship".
Good sportsmanship is an indication of good character.

I have always let them notice. In 25 years I had one guy sit literally for 45 minutes watching me pretend to think. When the clock hit about 10 seconds.. I made my move and punched the clock. He noticed after I punched the clock what happened. I was in a some what easy win for him. I called it my time swindle.

He didn't take it too bad though. It's your opponents responsibility to manage his clock not yours. Use every advantage you have to win. Even withholding information within legal limits.