This is almost exactly how I would see a girl yelling off her boyfriend for cheating on her ... in her dreams. As in, it has never happened in real life, she just dreamed of it, and got pissed at her boyfriend for it.
Like girl, you dreamed that up yourself, that's a you problem, not his.
I'd agree with you if not for three things.
1.) Sakurako is clearly aware of the meta-conventions of the romance genre, as well as her role as a childhood friend in a romance story, and clearly has been since chapter 1.
2.) Sakurako has pattern recognition. She saw how she fell in love with Natsuki, she saw how Sakaki started falling for Natsuki, and she's seen how close Aki's feelings towards Natsuki are to becoming a crush.
3.) She's not pissed off at him for cheating, or potentially cheating; she's pissed off at him for not thinking about how others would perceive his actions, romantically inclined or otherwise.
Sakurako's not perfect (she's a bit of a gremlin, and her antics keep relegating her to being a side character in her own manga), but she's not psycho, either.