D.M.
Only two suggestions. First, is the larger school a good school for her? If so, I would sit down with her and talk through the decision and all the options and considerations. Use it as a learning experience for her - as she will need to make many more decisions to come!
If the larger school isn't one that you feel comfortable with academically/environmentally, I would explain why she will be going to the charter school - it's in her best interest long term. Then plan on ways to continue her interaction with her friends. My stepdaughter has a friend (for several years now) that do NOT go to the same school.
Good luck.