Victims are best off if they can keep their own paper trail - ideally using personal lines of communication or printing them immediately so the company can't "lose" them. As I mentioned in another comment, even IM'ing a fellow employee a casual comment along the lines of "didn't that bother you when the boss used the N-word" or something. Their response likely confirms that it happened. Casual chat to your HRBP in the hallway? Follow up with an email clarifying the important points. May sound awkward if you haven't before, but it's common: just prefacing it with, "I just wanted to make sure we're on the same page about where we left things: <bullet point summary of discussion and outcome>".
In an HN discussion about dealing with medical billing / insurance people who screw you over, it was recommend to keep meticulous notes of everything, and that reading back specific dates and names of people you spoke to who contradicted the person you're speaking to now got people to take you seriously and fix their mistakes real quick. I have since found that to be true.
