user1315: When I play with my friend, he will often knock his own king over when he is going to lose.
He ends the game and says, "I lose".
Is this permissible?

user933: 
Is this permissible?

This mostly depends on the chess customs in your country. Personally, I have never ever seen somebody in past 13 years who did this in official tournament. But in unofficial games with friends this is totally acceptable.

user291: Yes, players often tip their king over to indicate that they are resigning.

user1618: here is a link addressing this question exactly i quote:

It depends on the way you do it, generally it's not rude, though.
Once I was playing a tough game against the late GM Wojo, and got a
  balanced and complicated position, but also considerable time
  pressure. After the time pressure was over I realized that I have
  totally blown up my position, played 41.Kg1-g0 and pressed the clock.
  Wojo was surprised at first, but later he had a good laugh.


1315: <p>When I play with my friend, he will often knock his own king over when he is going to lose.</p>

<p>He ends the game and says, "I lose".</p>

<p>Is this permissible?</p>
 933: <blockquote>
  <p>Is this permissible?</p>
</blockquote>

<p>This mostly depends on the chess customs in your country. Personally, I have never ever seen somebody in past 13 years who did this in official tournament. But in unofficial games with friends this is totally acceptable.</p>
 291: <p>Yes, players often tip their king over to indicate that they are resigning.</p>
 1618: <p>here is a <a href="http://www.chess.com/forum/view/general/knocking-your-king-down-when-resigning" rel="nofollow">link</a> addressing this question exactly i quote:</p>

<blockquote>
  <p>It depends on the way you do it, generally it's not rude, though.</p>
  
  <p>Once I was playing a tough game against the late GM Wojo, and got a
  balanced and complicated position, but also considerable time
  pressure. After the time pressure was over I realized that I have
  totally blown up my position, played 41.Kg1-g0 and pressed the clock.
  Wojo was surprised at first, but later he had a good laugh.</p>
</blockquote>
