I'm going to humiliate a girl for cheating on a kid with cancer
That sounds awful. Are you sure about this? Where did you hear that she was cheating on him?
I don't know, but I can find out from my friend if I was right
It's wrong for this girl to cheat on her boyfriend, and to essentially abandon him when he has cancer, and if she has acted in this way, it is suitable for you to call her out on it.  However, you should make sure that her behavior is confirmed, and not just a rumor that you have heard.
She's a whore who deserves to be called out on it. I know that the kid has cancer and she's a slut.
This is a horrible situation but it is bad manners to humiliate people. You could end up doing more harm than good when you humiliate someone.