would (have)

would have punched

If I were you, I would punch him in the face.

If I had been you, I would have punched him in the face.

Just to confirm: I was taught that it should be as written above. However, a friend told me that I was wrong.

And your answer confirms that I'm wrong.

I think it depends on the context of the sentence. If you're talking about a situation which has yet to happen, then

"If I were you, I would punch him in the face" is correct, since you are saying that if he does the offensive thing, then you would feel justified in punching him.

BUT if you're talking about something that has already happened, then

"If I had been you (or, if I were you), I would have punched him in the face" is correct.

Thanks.