I want to call a friend's girlfriend anorexic
Are you making fun of her? What makes you want to call her anorexic?
I noticed that around the gasping thinness and I'm thinking that she has an eating disorder
Anorexia is an illness, some people are really naturally skinny and can't help that. If you are concerned for them, you should get them help rather than labeling them so quickly.
"Some people", like those anorexic people.
It isn't right to make light of this situation. She may have a severe illness that can even lead to death, or you may simply be judging her incorrectly. Either way this should be something that is talked about with kindness because she could truly be struggling.