Yuki getting proverbially slapped in the face by Nanase like that has me tickled. To think that her devotion and "besties status" was that one-sided at the start when she was instrumental in fucking up Nanase's friendships in middle school makes me hope that Yuki's slide toward violent crashout is imminent.
I do think that this is as much a cry for help from Fuuko as anything, though I wouldn't be surprised if she herself didn't view it as such. Mashiro has fucked with her head for years; I suspect she's resigned herself to the revived toxicity of her senpai's affections once it was made clear by Yuni that Fuuko was cut off (blocking, the slap, etc).
It's telling that Yuni, who's always played that passive aggressive "woe is me" hand like with the secret "venting" socmed account, would simultaneously be mad at Fuuko for coming onto her and mad that she's not contacting Yuni and trying to make up. But I suspect this note and returned earring will be a wakeup call for her. She's already disillusioned with Nanase all over again, and in a way that feels more "sober" than the first time when the cheating with Fuuko began. I wager she was actually looking to confront Fuuko to some degree (in the very roundabout and passive aggressive manner that is Yuni Personified), but this is a hell of a wrench in her character arc spokes as she's trying to figure out what to do about it all.
But honestly I mostly just look forward to what happens when Yuni and Mashiro finally confront/meet one another. And who will have to save whom, from whom, when it happens.