Eri obviously wouldn't know the intricacies of the relationship, so it's fair to blame her for "being a homewrecker" the same way that kouhai did to her and Kyou, but to Fuyuki and Senpai, the relationship always had a time limit for however long Kyou and Eri were in a relationship. I'm sure senpai will have something to say about the development, but ultimately their relationship would have come to a close in that moment anyway, even if it hadn't already between the time of Fuyuki leaving Kyou and finding Eri.
Like she told Fuyuki where Eri was and then went home. Her entire purpose during the times she butted heads with Eri was her meddling to push the two together. She kissed Fuyuki while Eri was watching to make her jealous. Repeatedly tried to push her away because "I'm the one dating Fuyuki, not you." Again, to make her jealous. She was the one pushing for this outcome the whole time. It's a bit strange to push for their relationship over the one the series is actually about. The wedding at the start of the series being for Fuyuki and Senpai would certainly be a strange direction to take the series in, that's for sure.
And demonizing Eri for "using Kyou" to "run away from her feelings for Fuyuki" is REALLY weird, like have you not paid attention at all? If anything, Kyou was using her feelings for Fuyuki to get into a relationship with her "If you had been honest from the start I never would have gone out with you in the first place." She didn't know that her feelings for Fuyuki were romantic, they probably still aren't, she's just in a vulnerable spot and lonely and the only person she can rely on is Fuyuki. She's not running away from her feelings (you know, the ones she didn't even know she had,) these last few chapters have specifically her airing them out to herself (and thus, us, the audience) so I feel like people should be a little more aware of what's going on, but I guess an act of cheating really destroys any sympathy from the now-rage-blinded audience, huh.
