@dotawolfbrother: Foorcing someone to do something they don't want, and forcing someone to do something with the goal or scaring them for "revenge" is petty. Yeah, it's a "prank", nothing serious, but seriously, doing that to a friend is being  a dick. It's not because it didn't work that it wasn't a dick move to plan it. (I mean, even the narration say it's cruel...)
Also, i'm not talking about just this time, but she did this kind of things (planning a mean prank just for shit and giggles or for "revenge"; and forcing Sakurai to do things she know he hate) a lot. I'm not gonna reread all the chapter just to pinpoint exemples, I think you see what kind of moments I'm talking about.
About what you're saying "He just has the option to walk away". Yeah, I agree, but it's not that simple: she's pretty good at forcing him to do things, and in this case, he agreed because he saw she was mad at him, and didn't want her friend to be mad, so he accepted her condition because he think he did something bad to her. But he did not. And she know that. But she's still going to guilt trip him into a thing he don't like. 
But, I agree that "petty" can be a lot of thing, and maybe the word is too strong for what I meant. English is not my first language, so it's possible that i'm lacking vocabulary here. Same for "Lashing out", maybe it's not the right term to use. 
Still, she does a lot of things that are not really nice, and get  often forgiven a bit too easily.