Anorexics frequently struggle with opening up to and trusting people so it's unlikely she'll receive much of what you say/do until she knows you care.
Be her friend. Do not mention eating, food, her size, weight, appearance at all
, doing so will not help her. She is already hyperfocused on these things. Stay positive and upbeat.
I'm not sure how you can easily fit into the picture since she has a boyfriend who may not appreciate your "friendship". So support him in supporting her as a person...not as an "anorexic". Since she has already formed a relationship with him that may be your best avenue.
If he is not informing himself, be the one to inform him by providing information about anorexia and other eating disorders. Provide him also with information about local treatment options. She will probably value what he has to say especially if he is showing genuine concern gained from accurate information.
Above all, realize that you cannot fix this situation. Do your part but don't pressure either of them or you'll push them both away. Be available and be supportive. Our ears are often a greater asset than our mouths
Your heart is in the right place
. I'm glad you posted.