MummieHunnie,
I don't think anyone is talking about having a go, attacking her, ripping her head off, insulting her. Look, really really don't take this the wrong way, but it sounds like you're projecting your own issues onto the OP.
I have had an ED and have longterm issues with food. I grew up in a house where I was shouted at whilst I ate and where my pocket money was taken off me if I didn't lose weight and where I was made to run around the block after my dad at a certain pace to lose weight.
So I know about insults, etc related to weight.
What I think I & others are saying is that, the family is clearly unhappy already. The DD is getting bullied (as I was). Her mum is calling her a nasty name. Do you think that is making the DD happy? The parent's feelings are less important than the child's longterm psychological health which is currently being damaged. The DD is being bullied, insulted, treated in a superior manner, right now, by her own mum, amongst others.
So yes, I think talking gently and in a caring manner to the mum is not inappropriate. Seriously what do you think the DD will be like in 10 years' time if she goes on as she is?