If Nanase wasn't such a doormat, she would have princess carried Yuni (or at least supported her) immediately to the infirmary, justifying it by the fact she's the one who crashed into her.
It's just practice. Her teammates can live without her for the few minutes it takes to drop Yuni at the infirmary.
But no, Nanase lets herself be dragged around by the girls around her, effectively abandoning her girlfriend to the girl she knows had sex with her.
Not to say she deserves to be cucked (no one does), but she never really tried to understand Yuni needs and has her priorities all wrong. She clearly chose the wrong girlfriend. She needs someone less needy.
Yuni is just a pushover and Fuuko, the Caterpillar Fungus, is progressively taking control of her life.