Yeah, he has become a psychologically dependent on her already. I suppose it's not anything world breaking, though, even if she now left. In fact I'd say getting rejected by Gotou aided in developing that dependency. He had been aiming for her for years, which alone would have kept him motivated from day to day, but after the rejection, he most likely felt like he had nothing in his life anymore. Then Sayu appeared conveniently, and his life suddenly wasn't empty anymore.

I'd say Yoshida is now mentally healthier than he was before meeting Sayu. He also knows Sayu has to go back home. We can only hope the brother dude isn't any snake, even though I suspect he's a cold enough businessman to work for Apple, which is why Sayu could only be a loose end for him. He would probably like to avoid any scandals, so I don't see him cause trouble for Yoshida. Nevertheless, Sayu still needs to sort out her problems back home the official way. They all know that. Of course there's the slim chance something is wrong about her family far worse than I'd expect, but I didn't get the feeling from Sayu's actions so far that the brother would have been raping her or anything. I don't see how she'd even consider going back home under such circumstances. She would have most likely disappeared the moment Yoshida, in an earlier episode, said she should return home eventually.