It’s interesting how the world has changed. When I was growing up, I transferred into a school where I was the constant subject of racist remarks by students, and sometimes teachers. Back in the 70s, they were acceptable but it definitely affected me for many years following that. Fast forward decades and one of my kids attended an out-of-school enrichment class as a high school freshman. A couple of older boys from the rival high school started making comments first about her school, then about her attractiveness and race. The adult in charge just kept teaching. When I called the director of the program, she encouraged me to call the principal of their high school. I did and he handled it beautifully. He heard me out, promised to look into it, talked to the boys separately (they apparently admitted it), told them it was unacceptable, informed the parents and then he personally called my daughter to apologize and asked if she was ok. That was the end of it. I have always been thankful that he took such a reasonable approach and understood that a 14yo girl might feel threatened when a couple of 17yo boys make fun of her.
I really think your problem is your high school. They should have dealt with this better. That said, your daughter is a senior and this is NOT her friend. She should be told explicitly that people who express hateful, racist comments are not your friend even if you’re not the target of the comment that day.