I think it is human nature to have preferences. But people especially parents need to be aware of such preference and make a conscious choice not to.
I believe every kid is different, like in your friend's case. Instead of talking about looks and how the 2nd is different from the first, they could take the time to see what are the strengths of the 2nd.
It's ok to talk about differences but if that is the only thing that they talk about, as the child grows older, she will become self conscious and may have problems feeling confident of herself and be half guessing if her parents really love her. That'd be really sad.