It depends on the reason for the C. I would start by talking with the teacher for advice on how to improve and perhaps consider a tutor if she is struggling to master concepts.
One C is not the end of the world. It would be good if she could pull the grade up, however, as sophomore year grades are part of the UC GPA, so the C will affect that a bit. But if she brings it up this semester and keeps her other grades strong and takes rigorous classes, she can still end up with a fairly strong UC GPA.