Why wouldn’t the person using the N-word testify? I have an idea - it’d be stupid for him/her to testify. In your opinion, why wouldn’t that person testify?
1. The person can't avoid testifying if given a valid subpoena. In most circumstances, the person would not be asked. The relevant issues are what did the company know, when did it know about it, and what did it do about it. Also, perhaps, whether there was a pattern of this behavior across the organization.
The plaintiff won't call the person because the person can't help his case. The testimony by the plaintiff will be that he was subjected to this harassment; calling the person would only potentially lead to fact-muddying (e.g. the plaintiff called me a name first). The defense won't call the person because the defense's case will be something like, 'we responded to the complaint appropriately and with due care' and the people who can speak to those issues are the supervisors, the HR people, etc.
2. There are some conceivable circumstances in which the alleged foul-mouthed men might testify. For instance, if the company denies that anyone ever said the n-word. Maybe the alleged speaker would testify to say, "no, no, I said that he was bigger" or something like that. One could also imagine a circumstance in which the plaintiff calls the speaker to testify that he was instructed by management to racially harass the plaintiff.
But these are far-fetched hypos. They are also changing the fact pattern discussed here. The busy bee wants us to assume that the n-word was spoken by a black employee, aimed at a different employee, without being directed to do so by a supervisor. And if that's the case, the alleged speaker will likely have nothing to add.
3. The plaintiff could call a different employee to testify as to the hostility of the work environment. This employee could be of any race. A white guy could say, "oh, yeah, it was really bad. I couldn't believe what I was hearing," or "plaintiff was my friend and I could see how it was really hurting him to hear those words and be unable to do anything about it." The company could also call a different employee to testify as to the plaintiff's seeming indifference to the words, and that the concern with the foul language seemed to spring up after he was fired on other grounds.
What isn't going to happen is the n-word man take the stand and have to testify as to whether he is a racist. Nobody cares.