That questline is not part of the game. It doesn't work in latest update now. But if you are using older one here's how it turns out.
1.You meet them in the toilet. They stop sex. Boy goes out.
2.You leave and come back in the toilet again. You see her with Masuo. Tell him about what they were doing. He decides to have sex with her.
3.Find Masuo in street level hotel.
I remember it until this part.