So I would recommend ofc the basics like try to understand anorexia and body dys. ,so you can understand her better. Its not about the compliments u give her after she called herself fat for example and you say no you are not, its more like the timing when u need to say "your really skinny" it will have another effect on her mind. just compliment her at random moments and dont overdo it. if you find the right ammount you will do great and if it comes to the point that she is saying "im fat" for example dont disagree directly with a "no you are not". say something like "i find you perfect like that" and just express your opinion. Its reeally the liul things that will matter here.