A, B and C are in the hand. C is all in making A and B practically heads up. A bets and action is on B. Before B has acted A shows his cards to his neighbor who is not in the hand. D who is not in the hand invokes "show one show all".
1) How should the dealer proceed?
2) What does the etiquette say on this situation?
3) What do the rules say on this situation?