It's funny seeing people accuse the author of poor writing while ignoring the fact that the main character is a child and ignoring the fact the "bystander effect" exists. In this particular case he could act or the blonde girl could act; he is convinced the blonde will act and so he does not. Look up the death of Raymond Zach, a case where trained professionals (police and firefighters) thought the other group would intervene and let a man slowly drown in front of them. We all like to think we'd react in a emergency but that's not how people work. There's a lot of things that make no sense in this manga but MC's inaction here is not one of them.