I cannot offer legal advice, however, I can address the question from the perspective of conflict analysis and conflict management.

I am curious as to what your HR manager has to say about the situation. What was his/her reasoning for keeping this complaint under wraps? Is it possible that the complaining party requested that the complaint not be disclosed? I can see how HR could be caught in a bind between the interests of the two employees.

From my perspective, openness and honesty are best when dealing with conflict in the workplace. It is best to bring conflicts safely into the open and seek resolution. Since this employee was not aware of the complaint, he had no way of knowing that his behavior was offensive. Without this knowledge he was set up for a repeat performance.

It would have been better to address the complaint, even if the complaining employee was not involved in the discussion, in order to avoid future issues.

Without that kind of dialogue your employee may perceive that HR was documenting with the intention of building a "case" against him. And, this perception (along with the feeling of being betrayed and persecuted) can push an employees to the extreme -- workplace violence.