You are a good, caring friend. Perhaps you can invite her to lunch or coffee or simply go over to her home and in private discuss your concerns. She may not be happy about it and she may (or may not) fight it but at least you know you have expressed your love, concern and care.

You can even leave her a card with your feelings at the conclusion of your conversation so she remembers the discussion and that you care.

After that, it is up to her to do what she feels is best for her and her well being. Along with the others that are around her that are affected.

If her doctors are lax about Rx I should think they wouldnt take very kindly to a stranger calling sharing concerns. It might actually open a whole can of worms and be harmful for your friend in terms of future care.

As a last resort, if you were to witness her, clearly impaired and still insisting on driving, or actually driving, despite your pleas and offers to help, to spare others lives, you may have to call law enforcement. But I feel strongly only as a last resort. Would you calk the cops if you were driving around someone that was impaired anyway?

Regardless, you are coming from a good place. It may also help to ask yourself what would happen if the roles were reversed?