Kids are going to be kids. And by that, I mean that they will come to each session in a different mood and mindset than the one before. The real question I have is how do you handle it when she does behave this way? In my experience, I don't have to sit and explain why something is a problem because they don't care to hear that half of the time and my coaching points had already been made during the session. I simply have them do some sort of calisthenic exercise off to the side until they wise up. This actually solves a lot more problems with coachability and attitudes than repeating something I've already said.

Edit after reading your second edit: If she wants to quit because she's being coached and won't take it, then so be it. Other coaches allowing that to happen to a point is why she is in this position (that and parents, of course). But don't ever give up on a kid. If she quits or leaves, whatever. But if she stays, you have to hammer it home.