If you’re not African-American and have been raised in a mostly non-African-American community, you may have to re-read this a few times to start to understand the problem not only with her note and also with her apology note.
Both notes will look perfectly normal to you.
It’s like having straight hair with some hair sticking up in the back by the top, or a tag in the back of your shirt flipped up:
Other people can see it easily but you can’t.
Of course, you don’t have to notice it and you can get mad at others pointing it out to you. That’s always an option.