I don't think every.single.thing has to be a lesson. Sometimes a teenage girl needs to be a teenage girl. The occasional selfish act does not mean she's a selfish person, and I'm afraid if you try to force a charitable attitude on her, she'll rebel--because that's how teenagers operate.
If it were me, I'd make a bargain with her--IF she does something nice next weekend (volunteer somewhere, donate some of her belongings, etc), she can go to the fancy cinema when the movie comes out. If it's that important to her, I bet she'll do it, and feeling as if she has a voice in the matter could make a huge difference in her attitude.
But take that all with a grain of salt. I don't have kids, and I am also planning a massive outing for HP7. ;) I mean, it really IS a big deal... that's it, the final installment, there's no more after this movie is released. I'd be heartbroken if I couldn't make an event of it.