This is a multifaceted problem.
My 13 yr old dd (very sporty, extremely fit) suffered a head injury, at school, 4 months ago.
She has been very unwell, and has lost her place in her sports team. She trained hard for 4 years to achieve her place, and this has been a huge blow on so many levels.
She is gradually coming to terms with the fact that it will be many months before she rebuilds her strength.
We are concentrating on a good diet, plenty of rest, building fitness gently and gradually, and also concentrating on other, gentler hobbies such as music, that are helping to rebuild her confidence.
She has needed loads of support from me, and I am steeling myself to have another chat with the coach regarding the best way to proceed. I don't expect her to do this herself, as the coach is a bit inclined to think she should push herself, and she is not well enough to do that.
13 is a difficult age. Loads of moral and psychological support is needed, and also help to recognise what she can and cannot do, and kind and sympathetic understanding as she comes to terms with what is and is not realistic.
One thing that my dd has said to me is that she now appreciates how important it is to do everything to the best of her ability, (even if it means a change of direction) and make the most of life as things can change in an instant.
It is hard to come to terms with one's expectations being dashed, but your dd may find that focussing on other activities may turn out to be a good thing.
I would be very concerned about any worries about weight/dieting etc and if the coach is putting this into her head I would be very unhappy.
I wish you and your dd all the best and hope things work out.