And she'll be more apt to try and act in a manner that will cause you to continue to respect and like her. It's important that you mention specific instances in which you saw her treat one child unfairly. It's harder to deny the truth when it's presented with concrete examples. If your friend reacts with ambivalence about the child she's treating unfairly, as I suspect she will, I'd suggest that you urge her to seek professional help. In my experience, when a couple gets along harmoniously, and one of their children has a trait that reminds one parent of the other, that trait endears the child to the parent.
Parents playing favorites among their children is a particularly troublesome issue.
A young child is entitled to unconditional love, which is exactly what an unfavored child doesn't receive. Along with inflicting psychological damage, parental favoritism can imperil the whole family structure. He freed Joseph from the chores his other sons were expected to perform and provided him with more beautiful clothing. Indeed, this story reminds us that when parental love can't be taken for granted, sibling love also becomes endangered. For children can't grow up to love and appreciate one another if they feel that they're in competition for a finite supply of parental love.