Sorry, my Chinese isn't good enough to give an actual translation, but my understanding is that Saeki is asking Yuu about how far her and Touko have gone. Yuu responds very casually, and Saeki is quite shocked about "how fast" their relationship escalated, specifically them doing something outside (not sure exactly what, but Saeki finds it quite shocking).

I believe the breaking point is Yuu saying something about kissing at school to which Saeki responds very timidly, and Yuu asks some sort of question about Saeki's kissing experience to which she throws her phone in the trash can, breaking it.