I will try to explain myself in my rusty English. I think the reason Ruby stresses Blakeney's "betrayal" with sushi is because Ruby's criteria for making "real" friends is that they must be completely identical to her.
Blakeney is vegan, she likes books and studies english lit, she seems to be a "good girl" (not wild, no parties, no boys), respectful and (maybe) intelligent. And that's why Ruby freaks out when she discovers that Blakeney watches series like The Queen's Gambit, has eaten sushi, and so on. These are things Ruby didn't think Blakeney could do, because she has idealised her as her alter ego.
In short, Ruby should learn and be taught by people other than herself. This reminds me of the infamous Erimentha, who interrupted teachers and lectured everyone. If Ruby would learn to be humble, I think many of her quirks and obsessions would decrease. Living with people who are not her family does her more than good: they can help her to dismantle the rigid and toxic parameters that govern her life.