Granted I have absolutely no experience in this matter, but heres my take:

I wouldn't point out the errors unless the interviewer specifically asked me about it. Knowing what I did about the previous stage, I would assume that he had just given someone else the 'first stage' and they didn't pass, but he just left the white board up. If he asked I would be more then willing to point out the mistakes, but I would, by default, assume the interviewed knew all about the mistakes on the board and pointing it out would just be irritating.