Ideally, the advice goes as you should not stay with someone you are not happy with. But since your brother had a baby with the girl he now wants to part ways with, it is his responsibility to really reflect on his decision as there is a child involved here. He may decide to separate with the mother of the child but he must surely contribute to the well-being of the child financially and emotionally. It is his moral duty as a father to do this and also the right thing to do. Also, was he married to her? Or were they just seeing each other? Would be nice if you share more details - perhaps, he could re-think about it as well.