Answer: Watching your colleague self-destruct undoubtedly leaves you feeling sad and helpless. The wisest course of action depends on the nature of her job and the support offered by your employer.
If Natalie’s work affects the health and safety of others or gives her access to company funds, then you should share your concerns with someone in . Otherwise, you can simply encourage her to seek help.
When dealing with substance abuse issues, are an invaluable resource. If your organization has one, ask a counselor for advice on approaching Natalie. But if there is no EAP, offer her useful information without prying or becoming accusatory.
For example: "Natalie, you may feel it's none of my business, but I care about you, and I think you're having a difficult time right now. I'd like to suggest that you consider some professional help." Give her the number of your local mental health center or Narcotics Anonymous chapter, but don’t force any further conversation.
Regardless of how Natalie reacts, at least you’ve tried to help. As a co-worker, that’s about all you can do.
For general suggestions on giving feedback to co-workers, see How to Give Feedback without Criticism.