Technically, Yuni said something all the way back in chapter 3, before any cheating happened. But then yeah, she should've just broken up when the relationship still wasn't making her happy.
Like I said - I'm quickly losing any sympathy for Nanase as this goes on, because it really feels like she just wants to repeat history with Yuni even after everything that's happened.
I still think Yuni is the most at fault, because yes--she did say something to Nanase near the start. But clearly it wasn't enough, and rather than follow up, she continued to vent on her alt socmed account, pout and sulk, and ultimately give in to Fuuko while trying to hide everything from Nanase instead of just going "you know what? I'm not getting what I need from you, so 'goodbye' I'm going with Problem Girl, for better or worse".
Which is why I hope that Nanase actually breaks things off with Yuni before going to her friend Yuki or whatever, because otherwise she's also just a hypocrite which is something that will personally make me lose all respect for someone. (I think it'll make for better story drama so I do look forward to it here, though.) But I think all of them deserve to be unhappy, except surprisingly for Fuuko, because she actually seems like a victim of sorts at the moment in a way that's more serious than the others (given whatever history she has with Piano Glasses).