well if she's going to cast the spell no matter what we say then why ask? the spell will not work [saying some words in colourful sugar water might motivate you, but it won't change anything] as it has been said a thousand times before, what you [or your friend] asks for cannot be done by real spiritual magick, and it is dangerous.
if your friend really believes she's only beautiful if she is thin, tell her parents, a teacher, take her to a doctor. it is not a healthy frame of mind to be in at any age. a make over or a diet to get in shape is one thing, only valuing yourself if you are a certain weight is another.
if she is serious about weight loss, look into a diet and exercise routine. the two of you can do it for moral support. you could do a home spa day and makeover. it may sound cheesie but sometimes a new look and someone to talk to is needed. another something is to make two lists, one of the qualities she loves [her personality, her eyes] and one of things she doesn't [like the weight] if you notice the negative outweighs the positive, make some suggestions of things you like about her. once done, together, work on the list of negative things to improve. it won't be easy, nothing ever is, but it will be worth it when you reach your goal. remember; magick is merely something to help when things get tough, not a fix all.