She didn't get that because he never said anything. Why would she assume that he was lying when he accepted the confession because generally accepting a confession means that you're at least moderately interested in actually dating. (He accepted the confession by telling her let's date, he never told her he liked her so he never lied) He effectively did something just as bad if not worse than what those girls did to him. (Not really, his goal by pranking her was to find her true colors, of being not in love, which vice versa can also tell him if she really loves him, so ig his pranks atleast worked, if it was left to the girl, without the demon, they would've never made any progress since she was doing nothing) On top of that, the only reason that she went with him to help him summon a demon was because it would make him happy to summon one. She tried to learn about him and what he liked while he didn't even try to learn anything about her and made a snide comment when she indicated that didn't like something. Of course you would get mad if your boyfriend always acted like a complete ass for seemingly no reason. (Here you answer yourself with how she'll assume he was "lying" when he accepted her confession, "gee I wonder why my boyfriend is being an ass to me, maybe something is wrong? Maybe he doesn't like me? Welp anyways!") While he has his reasons, he didn't make any of them clear (yup this here is the problem which irked me, you yourself said the girl got mad because he was acting like an ass to her, but yet we never see her asking why he was doing so, forget asking she never even gives it a thought, even in her rage she's all about my feelings this my feelings that, when a simple "i really love you, so why are you treating me so badly" would have suffice), so she couldn't have possibly known that (ofc she wouldn't when she didn't even bother asking about the obvious mistreatment). She was extremely considerate for what she did know about him. (So much considerate that she couldn't even get that he didn't actually like her from his treatment)