General advice

Don't make promises you can't keep.
Accept and validate the feelings they have, rather than telling them how they should feel.
Encourage discussions about feelings (if they won't talk to you try to suggest someone else they might talk to).
Try to accept them for who they are, not how you think they should be; Let them be their own person, even if it means having different ideas from your own - often people with eating disorders have a very fragile sense of who they are.
Encourage non-food or weight related activities and discussions - try not to have lots of discussions about food or weight.
Look at your family's/flat's rules, behaviours, and beliefs around food.
Try to be consistent in family/flat approaches (i.e., ensure you are all saying the same thing to the person).
Try not to talk about food being good or bad.
Try not to talk about diets.
Let them maintain control over their life/activities and take responsibility for their actions as much as possible, and try not to make decisions for them (this will depend on how worried you are about her physical and mental health).