I've been accused of racism once (seriously, not Internet keyboard nonsense). It was completely unjustified, deeply upsetting and disturbed me for ages afterwards. Just so unfair and such a dreadful thing to accuse someone of (on a whim, obviously a reasonable accusation is not necessarily unfair or whimsical).
I have to say that I admire her relative calm. I wanted to commit violence, and I definitely shouted.
I think the 'slave' comment was completely unnecessary, and she hasn't helped herself at all. Before that, she was a victim, IMO. After that, people will apply the 'slave' comment and view the whole of the argument with the assumption she is racist. I do think her lack of fluency in English was a part of the issue. But I also think the guy was being desperately unfair to her.
Like my accuser, it's him that saw skin colour and made an assumption. There's an irony there.