Shiirn definitely brings a valid point to this. How you take care of yourself on the outside is a somewhat relatively good indicator for the people around you to at least have a good idea of how you view yourself.
Everyone is going to feel self-conscious about themselves whether it is about their weight, teeth, hair, nose or something. Failboat, I would ask the person that your friend likes to express something physical about himself that he feels incredibly uncomfortable about. Perhaps it would help your friend feel a little better and understand that everyone is going to have that little something about themselves that they don't like but to overcome that feeling and accept yourself will make you look even more attractive in others' eyes. I feel that obsessing over your insecurities will make you appear less attractive overall and less ready for any potential relationship. Like they say, if you can't love (or at least accept) yourself, how can your partner expect you to love them?
As for me, I don't have a particular body figure that I am particularly attracted with. I have been with obese, average and skinny men and I can tell you that while all of them were self-conscious to a degree, they were all motivated to either lose weight or gain weight in order to feel better about themselves and for me. As Shiirn mentioned, I knew that they weren't just fat (or skinny, lol) slobs or anything and that they actually were proactive about their lives and wanted to do things to continuously improve themselves and showed me what kind of person they were. Things like this are great in a person and tbh, is a turn on for me as well as others, I'm sure.
To be honest, some of my worst relationships were with men who were fit, muscular, and defined because they didn't feel like they had to do much to express themselves in creative or interesting ways and this in turn made the relationship feel incredibly bland and boring. I always laugh when I see them flopping in and out of relationships that only last a couple months and ask all their friends why they can't keep girls.
I'm sorry but while looks are still very important, they can only go so far. Good looks will hook you, but a good personality will keep you.
The world is not purely black and white.