have her see a doctor for some blood work to see how her electrolyte levels are. Then have her see a dietician who specializes in eating disorders and also have her see a psychiatrist and a therapist. All asap. The sooner treatment is found, the better.

I guess make sure you don't attack her when you confront her about this issue. Eating disorders are a very difficult issue to deal with. Also, don't make her feel like she has a big problem and try to keep life as normal as possible. I agree with akb about finding treatment ASAP, but you really need to find kind and compassionate people who really understand eating disorders. A lot of the times it is nice to have a doctor or team that has had an eating disorder and really knows what it feels like. Also don't make a big deal about weight gain. That should be the LAST thing on ur list. Work on having a better relationship with food.