More thoughts: you say that this is only once a month, so you just hope it will get better. At the same time, you dread her coming to live with you in a few years. She seems pretty angry now about her life and the way she is being raised (though entirely unable to say exactly what it is she’s angry about, of course)–she is legitimately unhappy with the way her parents treat her, and, being a child, she deals with it badly, by lashing out and demanding stuff, which is only encouraged and rewarded. She is only going to get angrier and act worse, unless something changes.
Unless your boyfriend makes some serious changes very soon in the way he parents his daughter, there is a trainwreck coming. He is not living up to his responsibility. There is little you can do to repair this girl’s life without his doing that, and soon it won’t be once a month, but all the time that you are dealing with this.