I'm really sorry to hear about your friend, waat. I think you did a very kind thing by calling 911 when you found him unconscious. If your friend was coherent and asked to leave, there is nothing the hospital could have done. People are permitted to refuse treatment. I hope someday your friend will reach out for help for himself.
Please know that nothing that is happening to him is your fault. I know how hard it is to watch while someone we care for is so self-destructive.