Guy doesn't act like an actual guy post-"transformation", more like a typical quirky "one of the guys" girl, just like from the "Sam" gender swap movie among others. Maybe that was intentional in order to play into the reveal, but it seems to be a common trope in these movies. Almost immediately after the "transformation" he starts getting flustered in cliche slo-mo eye gazing scenes with his friend/later love interest, but I suppose a magical change in sexual orientation could be explained the same way a physical transformation could, even though no physical transformation (actually) took place. They introduce a short-lived love triangle with another male character before defaulting to the archetypal "mr and mrs perfect childhood best friends", when the rival clearly had more personality, depth and potential for a relationship, showing more complexity and heart by miles, only to have his character make multiple fumbles. Of course he wasn't great either, being in on this scheme to manipulate mc's feelings to accept the identity of a woman, which then she displayed flawlessly by choosing the wrong guy in the end, who also went the extra mile to manipulate her identity and gain her affection.
Now about the transphobic undertones in the plot twist. Surprise! She was a "woman" all along (even while confidently identifying as a man and perceiving himself as such), so everyone basically came together to "fix" him by creating this "experimental therapy" to reinforce a delusion where he was able to see himself as visually male, before choosing a time to rip the bandaid off and "see" the "transformation" so that MC would then see it. A collective effort to alter his sense of identity through psychological manipulation in multiple ways, rather than helping him see the truth from the beginning so that he could navigate his gender identity authentically through mutual understanding and trust. Of course in the end he accepted the new identity, and now she, decided to live forward as a straight woman, but if only it were that simple. In reality, gender dysphoria and sexual orientation are not things you can just socially condition into people by manipulating their their self perception, and to romanticize this idea wrapped in a clique "best friends turned dream couple" story, I just found to be incredibly tone deaf to the transgender experience and disrespectful to the transgender identity.