Hum, I don't get why you say that? (I might be dumb) cardami
the first panels shows Oka seeing the person crying and that person seems to have longer hair than glass-dude had in the past. so it was probably the friend. it would also tie in with the whole "realized I will be alone forever" maybe that's when it hit him and why he cried
It’s so obvious the person crying in the library was his friend who literally told him he realized/decided he’d forever be alone…