M.P.
Is this a large enough company that his immediate supervisor wouldn't know why he was passed up? I would expect someone in management would talk with him about the promotion and on what basis they made the decision. Otherwise he and has coworkers would have this attitude which is not good for the company.
I would ask about this but I wouldn't go to HR. They didnt make the decision. I'd start with my immediate supervisor and ask who I should talk with. Keep the chain of command/responsibility.
I suggest saying that was a slap in the face as being a supportive statement. It means that person thought he should've gotten the promotion to me.
I suggest it's very important to have a positive attitude and approach this from the position he wants to do what's best for the company. He can say he is disappointed and wants to know how he can meet their expectations.