He dealt with it like a man... well, let's ignore the box for now, the important part is that Ryuusei had the backbone to go and tell that little yandere shit that he doesn't want to deal with her bullshit.
Now about the box and Giri's help... they are kids, having the help of someone is normal, you cant always expect that they can deal with all their problems alone and it is much more healthy to have someone by your side when you have an awkward/serious problem.
Give an F for the poor soul that entered the radar of the little shit.
On a more serious note, that girl may have a really distorted upbringing if she acts like that and truly thinks that she is right, as a teen her attitude is barely acceptable but as she gets older this can only worsen.
Also... @nobaka crying can be used as a manipulation tactic to make someone take back what they say or do whatever it takes to make the crying stop, usually giving in to the demand or will of the crying person.