Things to do:
1. Focus on the person behind the mask of an eating disorder or self injury.
EDs and SI almost always hide a much deeper pain. The issue is not whether they are eating or if they've hurt themselves but how they feel about themselves. Ask them how they are doing and mean it.
2. Be there
No, you don't have to be availiable 24/7, but you should let them know that they can come to you instead of skipping a meal, purging, hurting themselves, etc. I have friends who were availiable 24/7 duirng my lowest moments, who told me where they were at all times so I could find them if I needed to. Note that my "lowest moments" did not include being suicidall. If your friend is suicidal, you need to tell someone. Yes, that may be at the risk of losing the friendship, but it's better than you're friend trying to take their life. A few months ago, one of my friends was really depressed. I made myself availiable 24/7 to help her through a tough weekend, but when she started talking about suicide, I made the difficult decision to tell the Head Resident of her dorm. Suicidal threats, no matter how innocuous they may seem, should always be taken seriously.
Also, like I just said, I have been in the position of needing to be availiable 24/7 for someone for a period of three days. I know how emotionally and physically draining it is. 2am phone calls to help a friend go back to sleep are not fun. But, sometimes they are necessary.
3. Love unconditionally
A likely reason behind the ed/si behavior is feeling unloved and unworthy. The last thing someone needs is to feel like their friends and family are hating them as well. Say the words. Say, "I love you." Say, "I love you, not what you do or don't do, what you look like or what you say. I love you because you are my friend and nothing you do or say will ever make me stop loving you." Even when your actions say this, people struggling with this type of behaviors read all sorts of things into what you do. You say, "I like your hair today." and they hear, "You're hair looked aweful yesterday." You say, "I like that dress." and they hear, "I hate all your other clothes." It's best not to comment on appearence, unless it's something general like, "your smile just lights up your whole face!" or "I love the way your eyes twinkle."
4. Jump in and do things to help. Ask them what you can do, but often, they won't know what will make them feel better. Giving them a hug and saying, "you look like you could use a hug." or sending them a note that says, "just thinking about you. Hope you're day is going well." means a lot to someone strugglilng with severe depression.
Things not to do
1. Ask weight related questions.
Don't ask to know how much they weigh. Even if it's so you can say, "well, I weigh x, do I look fat?", they won't hear you. It's different for them. Some of my friends (whom I love dearly), said that. I was shocked that they looked so thin when they weighed quite a bit more than I did, but it didn't change my self-perception. I saw fat and was determined to get it off, despite what others said. Eating disorders are not about weight, so don't focus on it.
2. Watch them like a hawk
Don't ask to see their arms to see if they've self injured. Don't constantly ask them if they've eaten, what, when, and how much. It is ok to show some concern for the behaviors. If you notice them hiding out during a meal, it's ok to say, "hey, have you eaten today? Can I convince you to eat something?" Often, that was what it took for me to eat. I had friends who would sit with me during meal after meal talking me through a bowl of corn flakes or, at my worst, a small salad, convincing me to have a bite or two of their food inbetween bites of mine. If your friend seems to respond to this kind of help, great! Go for it. If not, try to find other ways to help, but don't force them to eat.
If the person told you about their behavior, you are (believe it or not), in a better position than if you suspect the behavior but they haven't told you anything. If they told you, please realize that it's not to hurt you. It is because they trust you and don't like hiding from you. Feel free to ask questions and offer to stand by them, remembering my tips above. If, however, you see marks on a friend's arm or notice unexplained weight loss and picking at food, you're in a much tougher position.
Tips if you suspect someone struggles with an ed or si
1. Set up an appointment to talk with them when you both have ample time.
2. Tell what you've seen, not what you suspect first. For example: "I've noticed that you've been losing weight and skipping meals. When you do eat, you seem to only eat veggetables. I'm worried that you might have an eating disorder, and I wanted you to know that I want to be here for you however I can be." This is a BAD example. Do not say, "I think you have an eating disorder because you've been losing weight and skipping meals. You should go to a therapist." It is important to encourage them to get into therapy, but not early on in the conversation. That puts them on the defensive.