This thread is timely, considering that February is eating disorders month. I just received a manual for our RME about eating disorders, signs and symptoms etc. I've also had friends who have had anorexia, so know how frustrating and helpless you can feel when you can't get through to them.
Eating disorders are symptomatic of a bigger issue, usually psychological. I have to echo AOX81 and say that first, she has to realize that she needs help. Getting her fiance on side may be a good idea, since he's the closest person to her. Does she have a pastor or another person in her life who's opinion she would listen to?
I've included a link to a local organization here in Toronto that deals with eating disorders. They've got fact sheets, signs and symptoms etc. The net has a lot of info on this topic, but make sure it's coming from a reliable source!
www.sheenasplace.org/main.html
You need to open a dialogue with her in an environment where she feels comfortable. Gently ask if she thinks she needs help. Mention that you've noticed changes in her that concern you, and suggest that a professional will be able to determine if she needs more help. Tell her you're here to support her and will help her find a counsellor or doctor in her area.
Please note that I am not a medical professional and strongly urge that your friend get immediate medical and psychological attention.