I would ask why she asked!!
I really don't understand woman. Those kinds of questions are loaded. It's obvious she already has issues with her looks and she thinks the other girl is prettier. If you tell her "no, you are just as pretty as the other girl", she won't believe you. Your friend has issues with self confidence. I don't know how you can help her.
Everyone has different body types and some people are heavier than others, this does NOT make one "prettier" than the other. Everybody has different preferences when it comes to what they find attractive. Unfortunately our society has done much to promote the very thin look as most attractive. I actually think woman's magazines have done more to harm young girl's self esteem than any men's magazine with barely clothed models (or nude) in the pages.
I look forward to reading responses from woman members. I am just as ignorant in this area as you are!
To secure the peace is to prepare for war. -Metallica