If it is indeed true that body image is one of the last things to change, imagine the experience of going through eating disorder treatment. Patients would, logically, want to feel better about their body so they can feel better about changing it, right? But it just doesn't work that way. So, what patients are asked to do is very hard indeed.
Patients are asked to put their appearance as a lower priority than their health, or their future goals. I wish I knew of a way to make it so they liked their body. And of course there are plenty of messages in our society that if we just change our body, we will be happy, healthier, probably even wealthier. What a misleading message. In reality, focusing on our bodies makes us unhappier and less healthy, both physically and psychologically. But, that simply is not how it seems in the moment.
Families and friends often come in, valiantly trying to rationalize with their loved one about their body. They try and try to convince their loved one that what they see in the mirror is not what is reality, and/or why do they have to care so much about their weight. And this is where it is important to be reminded that eating disorders are not rational diseases, and thus, unfortunately, rationalizing often has little to no benefit, and instead serves to frustrate everyone involved.
So, this bears the question: What do we do instead when our loved one is so upset about her body? Try to remain non-emotional and avoid the frustration. Assume what he/she is saying is what he/she really sees and/or believes, and put yourself in that position. It doesn't mean you agree with what they are saying or thinking about their body, and you can certainly say you do not agree. But, let them see you can understand, as best you can, just how much it hurts to believe what they believe.
Maybe ask them if there is any part of their body they do like; I often jokingly reference the big toe just to make the point that there is maybe some small part of their body they might not hate. And, yes, toenails has been an answer before. What about any part of their body they can value in some way other than appearance. For example, in my experience, many female athletes HATE their legs. They see their muscle as fat, or fear that others think their muscle is fat. But, they often can recognize they also value the strength of their muscular legs.
Also, help your loved one talk about whether they can make anything else more important than their appearance. Are there any situations where they forget, for even a second, about their body? Admittedly, these are often conversations that seem to go better in therapy than at home, so don't become upset if these topics don't seem to help.
Finally, look at yourself and things you perhaps unconsciously say or do. Be aware of whether you, or any other family or friends comment about your own appearance, or anyone else's. Work to change any family patterns around food and weight that unintentionally reinforce an eating disorder. Our society reinforces eating disorders ALL THE TIME, so of course families in our culture do. It's not about blaming the family, or even the culture; it's about understanding what change could help.
Body image concerns are so confusing to those who do not see themselves in such a critical and irrational way. But, coming to understand what your loved one does see can really help you support them in changing it.