While I disagree with your coworkers, I think as POC they’re entitled to have their own opinions on Rachel’s behavior. Based on your question and avi, I’m gonna assume you’re white. With that being said I think you should avoid attempting to correct them because they’re entitled to decide how they feel about her actions as POC. If you choose to say anything at all, I’d say something along the lines of, “I’ve read/heard numerous accounts from POC who feel like her behavior has taken opportunities from black women and trivialized their identity by reducing it to a tan and a curly wig. I tend to agree with them.” and leave it at that.

This is where “speak up not over” as an ally comes in. It’s not really your place to tell POC how to feel about someone co-opting blackness, but you can respectfully disagree while asserting that your beliefs are a result of listening to other POC.