She didn’t brainwash him, she put like a mental block on him so he wouldn’t have a breakdown and it was suppose to be temporary but things kept happening to him so she kept having to reinforce it. I believe that’s why he’s emotionless
I went back to look and I don’t think they meant they literally became friends with uichi I think there saying he’s the one there bullying now. And they don’t say it to rei, it’s something yotsuha over hears. But she does say rei is suspicious