Objectively, Kazuki did something very terrible especially considering that they’re friends. But I don’t think it is very far fetched from what a lot of people do. Many people wouldn’t hand over their designer sneakers to a barefoot homeless man on the street, nor would they give a Burberry scarf away casually to someone who needs it more.
I’m not even sure if Kazuki even understands what Moeka is going through. I suspect she thinks it was a roughhousing incident; we’re never shown Moeka’s family situation and Kazuki naively thinks that calling the police would help.
Also Kazuki wasn’t completely callous - she’s twisting and folding the handkerchief behind her back, but what ultimately pushes her to step away is her father calling her. It must have been jarring to step away from the glamorous party with a budding new romance to find your friend softly contemplating suicide. Kazuki’s childhood sweetheart is a good-looking rich heir, while Moeka’s ex-boyfriend is a shitty good for nothing bum. Being placed on the spot like that, at such a young age, I don’t see Kazuki’s decision to be unrealistic. I think she’ll ultimately learn how callous this was; this is the same person who picks dead pets to bury them with her own hands.
It’s interesting to note that Moeka did see the handkerchief; Kazuki was holding onto it while she was calling after her.