are they emo or depressed?

If one of your friends is cutting, tried to commit suicide and is dealing with lots of emotional issues, your friend needs help. And your friend is unlikely to search out that help. It's part of depression to want to hide and to think that the situation is normal. Please tell a parent or other responsible adult about your concerns. If none of the adults will do anything, tell a teacher. They're required by law to report dangerous situations.

After your friend starts to get help, there will likely be some anger about you sticking your nose in. After a short time, that will be replaced by gratitude. Just stick it out through the rough times and your friend will shortly realize you were acting in their best interest.

Emo means emotional. But people mix it up, they can be depressed, anxious, confused. It's a whole stage of things

If it's over something trivial or there's no reason for said behavior, they're emo. But if it's legitimate, then it's depression.

that means that they have depression and should seek help right away