I would think Eri still wants Harima, since she's visibly perturbed when he's handcuffed to Yakumo. As to why she would let Yakumo go with him, my guess is as good as anyone's. But as Yakumo put it, "this will really decide everything", so I'm guessing that's what Eri wants, to have end the whole issue decisively.
In which case the better question is, what does Eri hope to gain from that? Harima, presumably, but she can't know for sure that it isn't going to backfire on her. Especially when the fact is that Harima likes Yakumo way better than Eri, which he's said to her face more than once.
You make your face a mask. A mask to hide your face. A face to hide the pain. A pain that eats your heart. A heart nobody knows.