Ethical gut check: Do you keep a promotion not meant for you?

On the job that you do, the boss gives you a lateral raise with a lot more money. Late that evening he/she calls you and tells you that he/she is very sorry but they made a mistake in the promotion and that it should have gone to another. That in the morning he/she will call that person into the office and tell them, because they are somehow out of range and can’t be connected by phone. When you get to work, you heard buzz around the office that the boss was involved in a pile up, and died in the accident. No one knows the promotion was a mistake but you. Do you go to the person second in command and confess it, and say who should have had it, or do you dummy up and keep it? If the person who was supposed to get the promotion were a slacker, arrogant twit, an obnoxious SOB, etc, would that change the issue of if you would confess or not?