Do you have to give others a chance to say "Uno"?
From the original Uno rules:


  The game continues until a player has one card left. The moment a player has just one card they must yell “UNO!”. If they are caught not saying “Uno” by another player before any card has been played, the player must draw two new cards.


It doesn't explicitly specify that you must give someone a chance to say "Uno", though it does seem to advocate doing it as soon as you have one card left.

In my experience it has always fallen under the category of house rules as to how much time constitutes a "chance".  Agree on something.  If you can't, maybe ask "J" how much he likes playing Uno with you and if he would like to be invited back.