Bro, I generally agree with a lot of your posts, but you completely misinterpreted the messages and the dialogue, which is evident from you first message on the matter, where you said she "cheated".
She never cheated, and also what
HentaiKami implied about him being a "second option" also isn't even the case. It's simply an old fling she had, with someone who she considers a friend, and hadn't even seen in quite a while. They were never a couple, and she was no longer seeing him casually. Yes, she fucked up by not mentioning to the MC that she was going to meet up with him. However, her reasoning for meeting up with him wasn't because she wants a relationship with him, or that she has feelings for him. It was to tell him in person about her new situation, because she didn't want to do it over text, which is understanable imo. The only thing she fumbled was not mentioning this to the MC.
Also, going to the lake is irrelevant in this case, she was going there not just with him, but their other friends too. So yeah anyways, I'm only trying to set things straight, I don't actually give a shit about Simone, she kinda fugly tbh. However yeah, there was no cheating, or any plan on cheating. I personally have no interest in Simone, but I don't believe she's a bad person, she just made a mistake.