My friend went out with her friends ex

I have a friend named naomi that is very simple, she went out with her friends ex-boyfriend and the only reason why he broke up with her friend to go with naomi was because he knew naomi was very easy, everybody at school is mad at her and cursing ehr out for doing that to someone she calls her best friend, and naomi thinks she did nothing wrong, I tried telling her but she wouldnt listen, my question is what do you think of naomi, and what should I do???

I think she's right. She didn't do anything wrong. When people break up, that means they're free for whoever. Since the boyfriend was single, it doesn't really matter who he goes out with next.