To answer the question, yes an eating disorder can harm the baby, and sure as heck can hurt your gf - badly! The baby will be first to be fed then her which means nothing left for her! How long can a person live without food? You do the calculations.
Anyhow, my guess is she isn't pregnant if the disorder severe cause that would screw up her hormones totally and could make her unable to conceive for a limited time - or maybe forever?
I know this has nothing to do with your question, but considering the above things, don't you think the best you could do, even if it makes her mad, is get her help?