I am so sorry this is happening to her. I know you want to protect her and her feelings, but mom this is her battle. Soon she will be in college and will face even more difficult situations and you will not be there to "save the day", Your job right now is to help empower her, not save her. This is one of the most heartbreaking and frustrating parts of being a parent.
A similar thing happened to me when I was in a similar situation. A girl was saying I was a "token" Cheerleader because of a change in the elections. My mom encouraged ME to speak with the coach about the situation. The Coach reassured me, even without the changes, I would have been there, but the girl who was "suggesting" I would not have been there before the changes, was actually the one who go the new spot!
I then was able to tell others (only when they asked or said something) the truth.. The other young lady spoke with the coach and found out, I was the one with the correct information and boy, that stopped her cold in her tracks..
I suggest your DAUGHTER handle this. She is in High School and needs to learn to do this herself. This is her situation. She worked hard, She got the position fair and square and she should hold her head up high and either ignore all of it, speak with the coach, speak with someone in Administration at the school or speak directly to the girl or do all of it.
Help her think of ways to handle this. One thing I would encourage her to do, is tell this girl or anybody who questions her membership to this team, "If you have a question about my qualifications, speak to coach_______. "
Also have her speak with the coach , not so much for help, but to "inform her/him" that she is having to deal with this girl, but that your daughter is working on not letting the other girl get to her.
If the girl says something directly to your daughter, your daughter could say," I will be happy to have the Coach call you in and explain why you were not selected."
If it were to escalate, your daughter could also report this girl as a Bully. I am sure the school has a no bullying policy.
Let her know to not fall into these traps. Some people are just into drama. They are the little people in the world. Your daughter has a choice of joining in, or of ignoring those people and going on with her life, without getting pulled into this stuff.
I am sending you both strength.