Beauty is more than subjective, it depends of the environment, the education etc. but above all on History and culture.
In this specific case of black/white, here's my opinion.
When we used to be the dominent people (see for example, Kmt ancient "Egypt"), to be beautiful, spiritual, alive etc. equaled to be black.
For example, one of the prayer was saying something like "you are so dark that you are beautiful, you are so beautiful that you are dark".
At these times, being white was seen as awful, the color of death etc.
These ideas was kept after the dissolution of the Kemet Empire within the other African Empires /Kingdoms (i.e.Kongo kingdom, the word "black" in kikongo also means life and white represents the death).
When the African people met the white people (slavery), it has changed, the model was now the new dominant people.
Then to be white was to be beautiful, powerful, virgin etc. and dark beacame the death...every negative things.
When a people is more powerful thant other, he becomes the standars in intelligence, culture, value...and beauty.
If we were living in ancient times, your friend would have never said that.
Another thing that has an impact on our conception of beauty, the media (and our environment).
I don't know how it is in the US but here in France, you barely see Black men/women on tv.
The only you'll see are Beyoncé during the L'Oréal ads or other "African American" but very light skin-toned and of course with long straight hair.
It makes us think that to beautiful = being white or if you're black, look like Beyonce.
You must hv a very strong personnality and family/cultural backround not to be influenced by that.
I think your friend is just a victim of this propaganda, the darker you are, the uglier you are, the whiter you are, the more beautiful you are.
It's the same for Indian people, lots of women use enlightening creams to get "free" of this "horrible" dark skin and match as much as possible the standard of white people.
I think it' s the same for this story of thin/curved women.
