I've spent a long time deciding whether 'how to' really fits this post, because it's not some kind of guide or manual or step-by-step instruction. I considered 'ways to' or 'helping', but none of them really fit the idea I had in mind. Now that ramble is over, I'm going to say that obviously I'm not a medical professional, so take everything in this post with a pinch of salt and all that jazz.
Eating disorders are a form of mental illness that really do kill. With the pressure to be perfect coming from the media, they're on the rise, as we all start to internalise this need to be better, and it can get extreme. There are a whole variety of different types of eating disorder, which I'm not going to go into here, but this advice should hopefully work across the board as such.
Before I really get into this, I'm just finally going to say that anyone can be suffering with an eating disorder. There's a strong misconception that it's skinny, teenage, white girls who suffer, but anyone of any age, any race, any size and any sex can have an eating disorder.
Let's chat triggering. Whilst everyone has different triggers for different things, there are definitely some insensitive things you can chat about that can be really triggering. The whole 'urgh I ate SO MUCH for lunch today, what a pig' or detailing things you've eaten is just completely unnecessary. Be sensitive and remember that you don't need to account your intake of food to anyone to validate yourself. If your friend lets you know what triggers them, remember it, and don't shove that back in their face.
It's just not okay to bring up someone's ED either to their face or behind their back. It's not just rude, it's straight up nasty. If someone wants to open up to you about their ED, they will. Bringing up someone's mental health out of context can be hella upsetting and triggering. It can be really hard when you see a friend going downhill to not bring things up, but honestly, being supportive, and helping them where you can is much more helpful than raising the issue, unless it really is needed. Then, be kind and gentle, and remember that they're going through a really tough time.
On a similar note, if a friend asks you for help with things, give it. Go to the doctors with them when they don't want to go alone to talk about the ED, cook meals with them, do whatever they need because they're your friend and having someone's support when you're struggling can make all the damn difference.
Mostly what I'm trying to say is that your friend with an eating disorder is still your friend. Love them, help them and make sure they're okay, especially at this time of year, when everyone's attention seems to turn to food.