If there is a girl being mean to another boy saying he is worthless and that nobody wants him around what should I do?

If you're going to stand up for him, I wouldn't call her any names since that could get you into trouble.

same thing happened in my school, me and my friends just stood there awkwardly but I wish I would have done something

Tell her to mind her own goddamn business because everyone deserves respect, and if she can't do that keep her mouth shut. If you won't do that I would at least tell someone else because that isn't right.