You need to approach them carefully about getting help. You need to show them that you do care about them and want to see them get better. Most of the time they don't want someone forcing them to get help. Just be a friend and take it slowly.
Try to get that individual in for professional help. They will be able to tell if it is a chemical issue and may be able to medicate them so they can live a normal life. If they refuse help, you may have to leave.
Get them counseling. If they are self-destructive, they need more help than a friend or lover can provide - they need to see a professional to stop it.
