I don't think anyone in your daughters group needs to get involved or go against the other girl. If anything they should remain neutral. Your daughter will have to make it clear to them that they should be nice to her too. The other child feels discomfort with everything that happened, and feels that everyone is ganging up on her.
Your daughter should also put her foot down, and not allow the other girl to be bossy with her. You will have to teach her that. It would be nice to remain friends, but from a distance. They don't have to be too close. These things will happen again and again. They both need to open their circle of friends.
- the word love is too strong a word to use at that age.
-Please don't call a fifth grade dance a prom. That is ridiculous, it's just a dance which should not be happening, and if it does, the parents need to go with for supervision.
Good luck.