From what some people said in novel updates and what I remember from other sources some time ago, across the internet:
The girl didn’t initially believe it until she saw for herself, what seemed to be her boyfriend kissing another girl. Some people are the type to just simply dump their partner when they caught the act so it’s not that unbelievable, because that’s as good as solid evidence at least irl. The friend that ended up dating her was the culprit, and he was able to manipulate how information traveled, with also the support of whatever power he has. So that was how he was able to make her vulnerable and get to date her, and spreading the false information. The girl was still affected by what she thought was her now-ex-bf, that she hasn’t actually done anything with the new fucking asshole bf. So kinda like trauma from it so don’t worry, she’s still pure. Ngl she may as well just not date that asshole, I’m guessing it was the vulnerability that managed to land him at least the relationship in name. Due to how things went, the mc couldn’t get a chance to talk to her either, and as time went by it would basically get worse. Bla bla later on after the manipulation thing was over with, stuff happened and even when she came back for mc, it’s not like there’s no regrets or whatsoever. It seemed like she still can’t forgive herself but the mc was the one who wants to start anew. She also knows she has no place in his heart in that way anymore. Somewhat like a redemption development for her. Someone interpreted this as that since the MC wants to leave the past behind and move forward, then she should too. Would have been better to drag out the regrets first though so it doesn’t seem like it just went by easily, but I haven’t personally read it.
It was a little hard to tell whether what I found were credible enough because other people who gives low reviews seem to exaggerate or have dropped earlier on, and well, some say the earlier chapters are dragged out so you can take this with a grain of salt while leaning towards the reviews from people who read more into it. Like come on some actually came to the conclusion the mc actually cheated, or that the girl is the one just easily dating his friends.