Frankly, I can't blame Chiyo for being upset and hurt, although I'm not sure she even has the capacity to forgive since she, herself, is incapable of being open and honest with Igarashi. At least he was able to tell her himself about that night, instead of her finding it out from someone else. It's a tough situation for both of them. Igarashi needs to understand how he might feel if the situation was reversed and she had similarly spent the night with some guy while she was in Tokyo. Would he be able to believe that she had not done anything but sleep, or that the guy had not taken advantage of her while she was passsed out. Of course she'd never tell him, so there's no way he'd find out, anyway.