I just know there will be some geniuses so let's just get this out of the way.
Fuyuki is in the wrong here. She is in the wrong and she knows she's in the wrong. She never confessed her feelings to Eri fearing that it could ruin their relationship and prevent her from keeping her promise to always be there for Eri. She thought that by staying silent, she wouldn't hurt Eri like everyone else did but in the end, she buried her feelings so deep inside that she didn't notice just how much resentment due to Eri being completely oblivious to her feelings and dating other people festered inside of her. In the end, she became what she hated the most, another person that hurt Eri.
The manga isn't portraying Fuyuki as a victim here or right to be angry at Eri. Yes, Eri is insensitive and comphet af, but this chapter is in your face about the fact that Fuyuki is largely to blame for this fallout.