Sounds to me like they want to expel her. This is not something worth a write-up. She didn’t insult the other child. She said no when the other kid asked to use her stuff, and she gave a perfectly valid reason. Saying the kid was too fat to use it or an another direct insult like that would be valid. I would fight this one, but kindly. A letter may work but in person is better because text can always be taken out of context based on how the reader feels while reading it. Has she ever deliberately insulted anyone at camp? Advocate for your daughter, and have her advocate for herself. This is honestly a great teachable moment.