Nanase has a friend who is not Yuni. That’s it. Calling that emotionally cheating is pretty wierd!
Nanase also has a recurring habit of doing all of the things that Yuni wanted to do with her, with Yuki, and/or including Yuki in things that she does do with Yuni. Nanase effectively treated Yuki like a girlfriend more than she did Yuni in the majority of situations and incidents that we are shown throughout the series; Yuni's whole issue is that she is Nanase's girlfriend, but Nanase spares more emotional and mental bandwidth for her "friend" than for her.
You could argue it was unwitting on Nanase's part, but that's also a problem; we saw last chapter that she has a long-held habit of just not paying attention to things that aren't actually important to her in the moment, like how her best and closest friendship went back multiple years further than she assumed. She forgets Yuni more than once, and allows herself to be pulled around by Yuki or by volleyball or by her friendgroup at the expense of the person she wants to be her girlfriend. I'd argue she absolutely is emotionally cheating on Yuni with Yuki, and has been since the start of the series. Her being unaware of this, isn't an excuse on her part, just like Yuni's choice to repeatedly seek out Fuuko due to her frustrations with Nanase aren't excused by Nanase's negligence of her
when she could have talked to Nanase—something ironically paralled in her parting 'advice' to Nanase in this very chapter.
Yuni's a deeply flawed character who made bad decisions through the series, but they are informed by what Nanase, and then Fuuko, and then Yuki do, as well. You don't have to like or condone her actions, and I'd argue you're not supposed to even within the context of the plot as it's presented. But it is far, far less simple than you're painting it to be.
