Based Senpai. She's a great wingwoman and a friend for now, but frankly, I hope she'll have feelings for Fuyuki and will want to seriously claim her for herself. Much better than still being hang up about her fucking step father.
Listening to Eri, I'm thinking that it is possible to highly value a friendship but there's too much ambiguity here and codependancy for it to be healthy. For example, in Grey's Anatomy (scholarly reference i know), Meredith and Christina are like that, they are very clear that they are each other's 'person'.
But they still manage to pick their moments and know to leave room for their partners. When it works, every one is supposed to be on the same side.
Here, Eri is pretty much the only one not doing her part. She's antagonistic and jealous with Senpai, she's dismissive to her boyfriend and can't spend a moment with him without saying 'Fuyuki', and she can't even see how it hurt Fuyuki to shackle her with that promise, to expect her to see her dating whoever, when she's clearly in love with her.
Senpai is the perfect shock therapy for Eri to stop her denial.