I repeatedly told my friend to leave her emotionally draining relationship, but she won't!! What can I do?

Wisdom and knowledge comes from making bad mistakes. Life throws them at us and we have to learn them on our own. We never learn from other people's mistakes or them telling us what we should do (unfortunately). You're a good friend to try and help her but all you can do is keep encouraging her to do the right thing and be there for her. She will have to get tired of it herself and make the decision to end it.