I would probably pull her off that team. There is no,point in playing for a contentious coach, as he will be less interested in seeing her improve and be more interested in just being critical of her performance. It’s not, imo, a relationship that can improve. I would be inclined in getting her into something else that she can enjoy, or, put her in her chosen sport, but outside the school.
Besides, if she’s that bad, from the coach’s POV, he won’t miss her. He was also incredibly stupid to have sent anything less than the most professional of emails because it is now documented.
That’s the weird thing— unless my sister shows her daughter the e-mail, I don’t think niece has ever been made aware of any major issues and doesn’t dislike the coach. She still starts and gets a lot of playing time. What crossed my mind was what kind of e-mail is sent to those that don’t play all that much? Niece loves the sport.