It appears to be quite unethical for the human resource director to share this information openly. Yes, it should be confidential.
Has it caused harm to the employee? Was she not already off the premises? Resigning is far better than being fired - thus, it may not hinder her from securing another job.
However, if the employee was coerced into resigning, if the drug test was incorrect, if the employee did not violate company policy, then she may have grounds for a wrongful termination case, and perhaps other causes of actions (any such damages, emotional pain and suffering). If this is the case, then she should seek the advice and counsel of an attorney to protect her rights.
I hope this has helped. If you wish for further information, please let me know.
Peace, Love & Happiness,
The Mystic Wave
Information provided herein is based on my 30 years experience as a legal secretary/paralegal in the State of California, with experience and knowledge in the State of Nevada. This information is not intended to substitute for informed professional legal advice from a practicing, licensed attorney.