There is no problem here at all to begin with. People are latching onto similarities in this manga with the trans experience and how personal and serious that can be, even when Jun really doesn't seem to have had any strong identity issues at all. That doesn't really mean Jun's identity can't or won't change over time (I imagine it will), but people nevertheless ascribe that "seriousness" to it that I don't think it deserves. Jun's just kind of "comedy upset" at gyarukami, and I don't think people are really respecting that. They just see "she took away his exploration of a body type he was more comfortable with" and get mad at that instead of laughing. I think a more reasonable criticism would've been "they should've devoted a whole chapter to Jun exploring life in this particular form," because that is actually not an emotional criticism but rather a criticism of the narrative not exploring an interesting topic that it only briefly broached.
Anyway, is she acting immature? Sure. If any of this were real, would it be reasonable to step in as a third party and say "don't fucking change his choices without permission"? Sure. But I think it's okay for gyarukami to be completely immature about it, because this is a wholly different moral arena and it allows us to explore how Jun will react to that kind of a thing. Will he develop a grudge of sorts? Will he want to go back? Will those feelings mix with new feelings caused by his friends when he goes back to school, in case he ends up bigger than before? Those are all interesting things that I'm happy to have explored by the manga.
I have a problem, basically, with people saying, "it is wrong that the mangaka wrote this to happen this way"/"this shouldn't have happened as part of the narrative," which is the implication behind a lot of comments here. Like, yeah, it was a dick move, but I like the fact she performed that dick move, because it's going to lead to some interesting followup (potentially). One of the most interesting forms a narrative can take is characters coming to a positive conclusion (Jun realizing he is happier as a gyaru) through both positive circumstance (gyaru cameraderie, affirmations from family/friends, societal benefit) and negative circumstance alike (gyarukami messing with him, having his past essentially stolen, societal demerits). I do not mind stories where positive outcomes erupt from nothing but positive circumstance (the manga about a hero who pulled out a holy sword and became a girl, for example), but I think people are being too harsh on something that just contributes to the richness of the genre. Gyarukami's kind of an immature idiot, and that's okay. We don't have to get mad at her for having character flaws that put the protagonist in interesting situations.
