What if he hates me for telling? Why do people sometimes not understand that there are other ways of dealing with depression? I have done everything I know that I can do for this person. I've helped a few of my friends stop their cutting and stuff and I love listening and helping them and helping them figure out other soulutions for dealing with their emotions but I'm having trouble with this one

Do you think I should tell my friend's parents now?
Each person deals with and reacts to psychological issues, including depression, differently. It's not your fault that the other person isn't finding your approach helpful. Also remember that symptoms of depression include negative thinking and trouble focusing, so don't blame yourself.

Secondly, sometimes you have to tell another person what is going on when you are seriously concerned about another person's well-being. Tell someone you trust. This person should be able to listen and make good choices about what to do with the information you tell them. They should be able to understand that you want to respect this person's feelings and privacy and concerns about expressing these to anyone. However the person you tell should know whether or not it is a wise decision to intervene, and how.
Perhaps they could give you advice on how to better understand and help your depressed friend in a way you hadn't thought of before, provided the situation is not too serious.

If your friend is in danger, you need to tell someone.