I think what is interesting here is not that WIY is concerned that a Jewish girl is watching movies, but at the incongruity of a girl in BYBP watching movies.
Had the girl been in Flatbush Yeshiva, for example, would he be having the same reaction? It is still a frum girl, filling her head with the same garbage… But, perhaps such behavior is “expected” from certain frum girls and not others?
So, I don’t think it is the movie per se that is bothering WIY, as much as the hypocrisy of a BY girl doing something that a BY (and all those involved therein) is supposed to frown on.
So now the question can be rephrased as : Why is this girl doing something that her school (presumably) doesn’t allow? Where does this double standard come from? (And do the MO have something going for them, by way of less hypocrisy?)