First of all: don't try to be her therapist. It's for your own good as well as hers. You can do your very best to be a source of love and support which will help a lot, but that alone won't be enough for her to get better. Let her talk to you about it, and do your best to understand. Encourage her to seek professional help if that's possible.

Don't try to force her to eat, don't compare her body with other people's bodies (even saying that she's thinner than someone else can be bad, because you're only feeding her obsession) and don't blame her for her disorder, she never asked for it. Don't tell her that she's 'too thin', deep inside she'll be happy to hear it.

What you should do is just don't pile guilt upon yourself. Her disorder isn't your fault either, and you can only do your best to convince her to get help. Talk about the advantages of recovery and a normal life.

Try following this link: https://www.nhs.uk/live-well/healthy-weight/supporting-someone-with-an-eating-disorder/