The one thing is not to push eating on your friend.

There is probably something much deeper under her anorexia than just body image issues though that tends to be what it ende up looking like at the tip of the ice berg. The problem is that what is underneath takes good therapy treatment to crack & we as untrained just friends have no capability to even touch on the underlying issues....just be there & listen is about the best thing you can do. Don't encourage the behavior but also don't try to encourage eating as it can come across as forcing even though it's not what you are doing.

Just be there & listen & throw in a voice of reason if it feels appropriate & not accusing.

I'm sure others will have more suggestions.