user17865: In a "friendly" blitz game I got into an argument with my opponent in the following scenario:
My opponent's knight was pinned to his king. He did not notice and touched his knight to move it. I became excited and before he had a chance to press his clock, I took his king.
The question is: by official rules does my opponent lose the instant he touches the knight? (Making any action taken after this instant irrelevant?) Or does he need to "complete" the move by pressing the clock?

user17873: Taking the king is not a chess move, and putting your king en prise is an illegal move. 
I wish your rule was the rule. I could draw with Magnus Carlsen every time. I would touch my rook on my first rule, and as I have to move it and can't, therefore, I would walk away with a draw :-)
Touching a piece that cannot be moved such as the rook in my case, or the knight in your case, simply results in the touch being ignored. 
So in your case, you were mistaken: he simply needs to make a legal move with any piece and then press the clock. I believe had he completed the illegal move, you could have asked him to take it back, and get a time bonus in blitz.
However, touching the knight does not compel him to make that illegal move.

user12523: The touch-move rule never compels you to make an illegal move. It may compel you to make a stupid move, but not an illegal one. For example, you'd be forced to move a knight that's pinned against your queen and lose your queen as a result, but you'll never be compelled to move an absolutely pinned knight which would put you in check.
See this related question for more discussion, but I'll quote here the relevant FIDE rule here:

4.5 If none of the pieces touched in accordance with Article 4.3 or
  Article 4.4 can be moved or captured, the player may make any legal
  move


17865: <p>In a "friendly" blitz game I got into an argument with my opponent in the following scenario:</p>

<p>My opponent's knight was pinned to his king. He did not notice and touched his knight to move it. I became excited and before he had a chance to press his clock, I took his king.</p>

<p>The question is: by official rules does my opponent lose the instant he touches the knight? (Making any action taken after this instant irrelevant?) Or does he need to "complete" the move by pressing the clock?</p>
 17873: <p>Taking the king is not a chess move, and putting your king en prise is an illegal move. </p>

<p>I wish your rule was the rule. I could draw with Magnus Carlsen every time. I would touch my rook on my first rule, and as I have to move it and can't, therefore, I would walk away with a draw :-)</p>

<p>Touching a piece that cannot be moved such as the rook in my case, or the knight in your case, simply results in the touch being ignored. </p>

<p>So in your case, you were mistaken: he simply needs to make a legal move with any piece and then press the clock. I believe had he completed the illegal move, you could have asked him to take it back, and get a time bonus in blitz.</p>

<p>However, touching the knight does not compel him to make that illegal move.</p>
 12523: <p>The touch-move rule never compels you to make an illegal move. It may compel you to make a stupid move, but not an illegal one. For example, you'd be forced to move a knight that's pinned against your queen and lose your queen as a result, but you'll never be compelled to move an absolutely pinned knight which would put you in check.</p>

<p>See <a href="https://chess.stackexchange.com/questions/22616/is-this-considered-checkmate-or-not">this related question</a> for more discussion, but I'll quote here the relevant <a href="https://www.fide.com/component/handbook/?id=208&amp;view=article" rel="noreferrer">FIDE rule</a> here:</p>

<blockquote>
  <p>4.5 If none of the pieces touched in accordance with Article 4.3 or
  Article 4.4 can be moved or captured, the player may make any legal
  move</p>
</blockquote>
