Hey man this is a really delicate illness and the best way to help would be to try and boost her self confidence. I would also speak to her and tell her that its important that she tells her mum and if she won't then say you are going to tell her yourself, this hopefully will make her speak to her mum rather than having you say it (hope that made sense).
Don't ever say anything negative, even if you think it might help. for example - telling her skinny girls are unattractive, because she will know she is thin and will then just think she isn't good enough.
PM if you have any questions I'd be more than happy to help