Y'know. For the actual history they have, this reaction totally fits. Like, yeah, it's not like Sumire did anything particularly cruel or abusive, they just broke up. It sucked, but that's all it was. Naturally, if senpai shows up again, and acts all cool and charismatic and flirty, she's going to be swayed. Totally. Sure.
Now, if only Izumi hadn't acted like a violent weirdo for the last ten chapters, ostensibly because she's just that hurt because of what this woman did to her, then the fact that she's immediately turning to goo after spending five minutes alone with her would be fine. If her grudge held any weight whatsoever, even if it was utterly undeserved, then it would at least reflect some deeper pain on her part, but as it turns out, even she didn't care about it that much. She was just using her mildly unpleasant experience as an excuse to be an awful person. That's amazing.
God. I think the only way I can see this actually going well, is if all three of them get together, Izumi has a total meltdown, reveals their "dark" past, and Sumire's just like, "Yeah, she was always like this. That's the actual reason I broke up with her. I thought she'd gotten better, but she's actually even worse than before? Damn. But hey, you're pretty cute, let's go for a drink." And then Maki and Sumire ride off into the sunset and have an actual healthy relationship where nobody gets treated as a punching bag. The end.