is the part where she says he was the only one faking it a bit weirdly translated? does she mean that she wasn't actively participating in faking it? cause otherwise it doesn't make much sense with what he says after
加藤君が勝手にそうしてただけでしょ
I think this sentence fits more with something like "Kato-kun just did it on his own" so I think the meaning is more like, he was the only one pretending because he was the only one doing the show, she was just following along but and not doing anything for show.
idk if I made myself clear XDDD but I I guess it got confused because its a hard sentence to translate.
It's a kind of classic dialogue that an extrovert pushes himself, dictates the scenario and invites you to follow along, and she doesnt rly like the idea but goes along anyways because w/e, sounds benefitial.