Well, he wasn't looking through a window, they were outside, with umbrellas over the tables. And it looks like the Yuzurihara is holding his phone in his hand. So, perhaps the original abusive seme called him there as a set up to witness him with Yunoki, or Yuzarihara thought he was meeting or has been meeting the original abusive seme. ..Like is Yuzurihara's expression shock at seeing Yunoki with the abuser or shock at being caught? I think we are supposed to wonder what the truth of it is , just like the two victims are wondering right now.. just my opinion.
Or maybe he placed a tracker on the other guy's phone before he returned it.
I honestly don't know whats more upsetting on page 30. The scary face that the abusive scum is making? Or the face of betrayal the glasses seme was making watching them from the window. But yall know what that means shits about to hit the fan. *crosses fingers* glasses seme plz go and kick abusive scums ass. PLEASE! before we all have to go in this book and do it ourselves