I don't know what you should do but I'm sorry you're in this horrible situation with this woman. I don't understand how she can make friends at all! What if you talked to that HR lady as a person, not with a formal complaint? Say "I'm concerned about your friend". Which is true! I don't know what else to suggest except going to the head of HR like someone else suggested. The woman harassing you has too much power which doesn't make sense even if she's chummy with the HR lady.