mentality

I think the only way to stop her from behaving like that is by asking her that"Love yourself where there is love there is always miracles"So that she can able to except herself and feel better.

This happens due to lack of self judgement. Extrovert girls always sees on others and do not analyse what she is. So this type of thinking comes in mind. God has given us a fixed shape, body etc which we should accept and live accordingly. When we see other person one by one, we will find one is better than other and so on. Why should we waste our precious time on such silly matter. I think your gf's mental condition is not healthy. Need some help of expert.

self condemnation is the worst thinking one can do. This negative thinking will sometime make the unreal and real. Therefore one should desist from such thinking and always think positive.

Her friends can change her attitude by encouraging her. They must highlight that her positiveness is greater than her physical apperance. After all physical beauty will vanish gradually.