As a mother I wouldnt get involved. It will embarrass her! Talk to your daughter about how to handle it on her own. Help her with what to do and say to her friends. It will help her feel confident now and in any future issues.
Tell your daughter that a friend doesnt keep secrets they tell u secrets and then u should recommend her into something like ballet ,cheerleading or soccor or something where she can meet new friends and then she will find and make better friends that are good friends for her
you better teach her to fight back or she'll be treated /bullied the rest of her life. I know we're supposed to teach our children to be passive. Does it really work? My kids aren't allowed to start anything they're allowed to finish it and I've got their backs.
Reassure her that her friends don't know any secrets about her. Or caution her not to tell her secrets to other people. Possibly try to find out what the secrets are, in case they are something a parent should know. If she means simply that her friends are talking behind her back. advise that she find different friends. In any case, give her the reassurance that she is loved no matter what people might tell as secrets.