'The die is cast' which means 'decided finally'. In a game of die, if the die is cast, there is no chance to revert it back or call it back, and the situation is completely out of the control of the plater then. Hence, the expression means a situation when something is done finally and cannot be undone.
The die is cast there is no point of discussing over it now.
His decision was right or wrong, but the die is cast in case of conclusion.