All this 'it's from another culture so you can't judge' is just bollocks tbh.
DH is from another country, as the youngest (surprise?) addition to a family of 5 his parents decided they couldn't cope and farmed him out as a baby to an older woman, a distant relative.
Perhaps she loved him in her way, but one of his brothers has let slip that he 'wasn't allowed to play'. He didn't return home until about 10, when his oldest brother (teenager at the time) protested to his parents that it wasn't right.
I see first hand how his upbringing has affected him, the lack of confidence etc and always feeling his is on the edge of his family. He doesn't like it if I say anything about it though, as criticising the family is a big no no apparently. So I don't. But it's fairly obvious.
I think it's cruel. And yes the English upperclasses sending their kids off to boarding school at 7 is also a terrible thing, a lot of upper class men are emotionally repressed and completely messed up because of it. This happened as a child to Stephen Fry and I noted that in his autobiography he defends his parents' decision, saying that's just what they did in their social circle. He had a lot of unhappiness at school, was seriously sexually assaulted by an older boy, again he glosses over it and says he's fine. I doubt it though.
I also had an Eastern European friend living in the UK, she was pregnant with no 2 and had health complications. So she sent her 2 year old back to the home country to be cared for by the paternal grandparents for a while. The child ended up in hospital over there, meningitis. I do wonder if the shock of separation from his parents caused his resistance to disease to be lowered. Afterwards my friend bitterly regretted her decision.
We have known for decades how separation from the parents especially at a very young age affects a child, it's in the Psychology A level syllabus FGS. So why is this to be dismissed just because 'it's a different culture'?
I would have to say something to your friend OP, even if it means the end of the friendship. Perhaps she has never been challenged on this before, so never questioned the thinking.