@bigsnek
Just like
@Kanilla stated, the one saying that was Takagi, the girl whit black hair, as she now knows that Taneda is not only is "dating" (intentionally not clear) Arima, he is also extremely possessive and jealous, and that he is basically a pervert in her eyes now, therefor she shattered this fantasy the girls have about Taneda, therefor she lost any interest he had in Taneda.
The nature of the relationship between Arima and Taneda is left on the open, probably intentionally, as we are basically watching the story by the eyes of Takagi, meaning that we got the same impression as her, that Taneda was forcing Arima and also manipulating him.
Intentionally, we will never know the nature of they relationship, but given that Arima keeps meeting whit Taneda despite what happened, either they just have the usual "bully seme x gloomy uke" and Arima is the cliche "I dont like it but I also dont not like it", or Taneda is somehow forcing Arima to do is biding, we will never know I guess.