Given this is less than 10 chapters in length, Nina's proactive decision making here is only appropriate.
But for her character, it's also encouraging--because it means she's not going to take this destruction of the Mao-senpai in her head & heart lying down. Just in terms of her getting a shot at her happy ending, this is good news and a good development for her.
I am curious as to what happened to Mao in the span of time that Nina hadn't been around--she went from this quiet, but clearly self-possessed individual to someone who changed everything about herself for the sake of a guy who is clearly with multiple women simultaneously, and doesn't care about "getting details correct" in compartmentalizing those relationships. Or, worse, not where Mao specifically is concerned.
So something must have happened to shake her and shape her into the woman before Nina today. It's just....quite the shift, and not the sort of regression of identity you'd expect when it seems like someone's gotten it "figured out" in the middle of adolescence. And if that is the case, then the question becomes whether Nino can do something to affect change again.
Whether or not Mao can, or more importantly wants, to become like she used to be, is of course still unknown. But I'd trust Nina to be cognizant of whether her friend and senpai is happy, and to push toward making her so if she isn't.
Nina's outrage boiling over is reassuring, though, because if I didn't know this was a short-form series I'd worry she would just roll over and scream in her head but ultimately never make a move. Never have I been so happy to be proven wrong, though.