Hi,
the weight does vary depending on how long they've been sick, etc. Like Vikki said, I can stop eating for an entire week and because my body is so messed up I will only lose a few pounds, but someone else who is healthy will stop eating for 2 days and lose 10 pounds. It all just depends.
Also, you should remember that ED's, although characterized by weight, are really not about weight at all. Myself, personally, am "normal" weight for my height, but I've struggled with bulimia for years and I've even had points where I would be considered anorexic. It's important that if you are afraid for your friend that you watch her or talk to her about this. Becareful not to be too confrontational, she's probably very scared and if you can help her early on in this disorder that would be amazing!

good luck!!!
NEVER GIVE UP
~sarah~