My immediate thought would be that you really have to talk to the parents about it.
You should probably meet without the children around.
If you explain calmly that you understand this is the way it is done their country, it is just not the way it is done over here, she should understand.
Does their child ever come over and play at your house? or do they only want to play at the friend's house. If they only want to play over there it is probably because they are being left alone so they can do whatever they like.