Considering the meeting at Yuzu's apartment was in April and the coaching arrangement with Boyang is announced in late May, we cannot say that Yuzu's not being notified beforehand, isn't it?
Also, one of the thing that I admire so much about Yuzu is his humbleness. With the way he is, it is hard to imagine that he will feel disrespected or any of that. Let's also remember that Brian as a professional coach has right to take on other students.
When this new arrangement proves to be not satisfactory for Yuzu or Brian in the future, they can always talk it out. But it is a possibility and not a certainty, so just leave it for now.