I have a really beautiful friend, but she thinks she is ugly? Why is she like this?

I have a really beautiful friend. I don’t think she knows she is beautiful though. It is as if she wants to be ugly, she just makes herself look ugly. She wears really big oversized, black, jackets even in summer! And most of them have holes in them and are torn. And she has two pairs of pants and shoes that she wears everyday! She always wears her hair in a ponytail and never wears any make-up. One time we mad her hair loose and put make up on her and put her in some nice clothes. She looked so beautiful, she looked like a model! Like she had walked off a vogue cover. When we told her to look in the mirror she just glanced and said no it looks bad, and wanted to tie up her hair etc. She’s pretty even when she looks how she normally looks but if she just does a little more, she looks gorgeous! I don’t understand how anyone could like her and not see that in the mirror? And not want to dress up everyday. I wish I could look like her. Numerous times she’s told me how she hated herself and how fat she was, but she’s not even fat! Why is she like this? Most of my friends spend hours trying to look prettier, thinner… and she spends hours trying to look bad. She’s even told me once she wishes she had a mask that she could hide behind. Why is her self esteem so low when she is so beautiful and everyone that sees her says so?