Here's why:
* Pronoun Case: "Her" and "I" are in the nominative case, which is used for the subject of a verb. In this sentence, "her" and "me" are the objects of the preposition "for," so they should be in the objective case.
* Object of the Preposition: The preposition "for" shows the direction of the cheering. The crowd is cheering *for* someone, making "her" and "me" the objects of the preposition.