This is what I understood from her reaction: “So he considered me his girlfriend?”
I don't think she felt the same way about him, but now, seeing how well he's doing, seeing what she's lost, she'll start to wonder “what if?”, what if they were boyfriend and girlfriend, there's nothing wrong in that, they're great friends, the sex is great, he's a great guy... what if?
At first, she admitted, then felt guilty and realized how bad she was without him. It's hapenning before the scene with Bart, when he cames to see her, she tells him that riding the waves is not her dream, it was a mistake, she spent all money and she miss her home a lot. She made a mistake by listening to Bart, who gave her an advice to "ghost" her boyfriend as he did to Emma before.
Scene on bench in park says a lot about Zoey and MC as well, and what he came through, when she ghosted him several months ago. Scene is really very strong. Someone is trying to play alpha-males and jokes that MC is a sissy, that he'd better would tell her to fuck off, etc. But either this desire to seem cool in public, or someone did not have real friends who really meant something in their lives (or both). True friends is not just buddies with whom sometimes you pass the time. Considering what MC went through at school, and since it wasn't his fault in many ways, and who Zoey became for him, it's safe to say that this scene and their backstory is very plausible and humanly touching. Zoye's words that she is afraid to ask MC about anything, because she is afraid of his answer, implying that he will tell her to go to hell, touch a nerve.
MC's words about how he tried to forget her after she stopped communicating with him. How he arranged a memory walk to the places where they had been and spent time together, after which he decided to move on. It says a lot about his feelings for her, and that even if they didn't discussed it before she left, both considered it more than just a frienship.
MC told her about "ex" thing without any second thoughts. Because he really considered her as his girlfriend, even if he never told her that. And her reaction told a lot and shows how she was surprised to hear that. Simultaneous joy and sadness from the realization of this.
He also told DIKs that she is his ex. He told the same a bunch of girls as well. Sage, Quinn, J&M. Bella also heard that, since they played drinking game together. Only Jill was not filled in. Knowing this, dialogue option with JB seems strange to me. If it would a some kind of major choice before Zoey arrived to the party, like "what to tell", since after that it doesn't have any sence. Telling different things to DIKs brothers, Derek and girls is silly.
Seeing her arrive “late” in the game, I imagine that her role is to be a temptation to the MC, and a threat to the other girls, based on the prediction of the number of chapters the game should only be 60% complete I believe, so there is still a lot to happen.
She will try to get closer to him again, try to stick together like old times, comfort him when he has setbacks, she will be there for him if he encounters problems in the relationship (and MC is a crybaby, that will happen), and if they have sex again, she will show that she is also an option, she is interested, she will be a threat to the other girls.
Right now, that's the only role I see for her in the story.
She arrived just in time. Right after "crossroads" Episode, when LI choice was made. It creates additional and potential doubts and temptations.
Telling her that you're still mad on her cools things down with her a lot. MC is being pretty cool with her. It reflects in their phone chat, costume contest and finally he refused to share a drink with her. On Others branch it also mean that MC doesn't offer her to crash in his room.
Up to the moment, J&M was the only ones, with whom MC frankly discussed Zoey and their relationships. The rest of MGs will be filled in EP10, probably.
Cheat points / temptations will come into play with Zoey, obviously. We are in "safe" zone only being in Others branch. I'm sure it's not necessary to be mad on Zoey to hold her apart. They wasn't able to talk at the party about MCs current love life, so they, probably, would discuss it soon. Such potential dialogues and options in them could be important. It's how I see it at the moment.