I do understand your reasoning and I'm not sure about the cultural differences, but my grandmother's generation (from a very rural area), would've handled it the same way. Confront, beat some sense into the guy, if necessary get his mother to smack him (that always works like a charm, no matter the age). Ofc that backfired sometimes as well, but - yeah, that's a "grandma-style"-reaction (at least by my experience). Those huge apartment complexes kind of derailed into their own small worlds with codes of conduct no longer matching "grandma-style". But that's just my take on it. ~
The problem is we don't know what Granny said. And unless she thinks she has some real way to influence the situation and to back up whatever she said, then she should realize that she is likely sending that kid home to an angry and abusive father. I think in her place I might have asked around some of the other neighbors first, and even scolded them on their attitude, since they seem to be well aware of the situation. Then with some more info as to what was going on figure out how best to handle that dad.
I know why people are talking about the shitty abusive dad, and the poor decisions of the mother (I´m not criticizing her, and know it´s difficult for her but her decisions doesn't stop being poor) or how xiao hang it´s an asshole, what I don't understand is how no one it´s talking about the super poor decisions of the granny, I know she probably had the best intentions and all, but what she did it´s so wrong in so many levels, and it seems she did that just for the sake of her own ego cause thats no way to help someone in need, especially if you suspect he/her suffer abuse (any kind of abuse), you can just make things worse or more difficult for them, if you can't or don't want offer real help don't poke your nose where it´s not needed, granny is so wrong in that situation.