Unless you have a union contract specifying that this is required, the answer is no; you do not have to allow this.

However, seeing as how this person sounds like a potentially problematic type of individual, I think you would be wise to have a human resource professional or a neutral member of management sit in on the review. That way it cannot turn into a he said/she said kind of situation. Everyone tends to be on his or her best behavior when a third party is observing.