Hi fellows,
First of all I hate “action changed”. If a player acts out of turn it should stand. That will make them start paying attention and make them quit trying to draw players in.
Now, in this case B acted out of turn. He should have known that A hadn’t checked or bet. He should pay attention. B acted in turn, be it probably slow.
Player A’s action should stand and B should complete or fold.
The amounts don’t really don’t enter into the case but completing to 15K shouldn’t change the rest of the play of the hand.
Good Luck,
Martin