Well, it's not the first time in manga that someone see bullying and don't say anything nor try to stop it. Also, she also see Ai hugging Hina afterward so she can still question if she was bullying or doing some kinky play, bullies don't really hug their victims like that. And anyway, it's technically true that Hina in her current state seem a bit unstable,even if she doesn't consent most of the time, she still agree to go with Ai's antics.