First of all, thanks for posting! I know it can feel very frustrating and helpless when someone we care about is suffering and we don't know how to help.
I posted a response in another thread that may be helpful here:
I think the answer to this question is very personal and depends person to person, but in general--with any issues not just eating disorders--the best way to help someone is to listen, to be present and there for them, to be supportive, to validate their experience, to provide resources and suggest more intensive help if appropriate/indicated BUT to also know that you can't force help on anyone it has to be up to them and treatment doesn't work if someone isn't motivated from their own desires to get better. Know that some people are in a place where they want to get better, and some people aren't. That's okay. Motivation and commitment ebb and flow and ambivalence (wanting to get better but also simultaneously wanting to not give up the illness) is totally normal. You can also ask them what type of support they personally find helpful and go from there. Eating disorders are hard and exhausting and complex--if someone is looking for support with an eating disorder, they don't want to hear things like "well just eat more" or "just eat less and then you won't have to purge" or "just stop throwing up after you eat and you'll be fine! You don't need to do that!" We know what we're doing, we often know it is harmful, and if we could "just stop" we would. Don't try to fix the situation or cure them, just be there and be supportive and open to what they need to say.
Also, as it sounds like this is something you haven't spoken with them about before, try to bring up the topic gently--avoid using accusatory or judgemental language and make sure to approach the topic in a comfortable, private setting. You could try saying something like "Hey, I hope this doesn't come off the wrong way, but I'm really worried about you. I don't know if anything is going on, and we don't have to talk about it, but I want you to know that we can talk about it if and when you are comfortable and I'm here to support you through this in any way I can"
Support can look so many ways--it can look like encouraging them to see a therapist, it can look like not making plans to hang out involving eating out in big groups, it can look like checking in on them when you notice a behavior being used, it can involve offering to eat together in a private setting for support and accountability, it can look like sitting in silence, it can look like acknowledging how hard they are working to cope.
Thanks for being open to learning, and acknowledging areas that you don't understand. Feel free to reach out with any questions of if you want clarity on anything!