I also don't think you have a legal case, unless perhaps your soon-to-be-ex will swear in court that he got the information directly from the pharmacist's assistant, and even then you would have to prove that you were somehow damaged by it.
However, you still have rights as a consumer. Complain! Complain to her supervisor, her supervisor's boss, and even the president of the company! Question them: what is their company policy on protecting customer's private information? Do they even have one? How do they enforce it? What kind of training do they give their employees? You get the idea. It is actually in their best interests for you to bring this to their attention. Because if it turns out you or someone else actually COULD prove your case, THEY as her employers would be held responsible for her actions.
You never know--she might already have complaints against her from other customers for the very same thing.
But you must protect yourself from being sued for liable or slander. Since you can't actually prove she did it, you can't actually say she did it. However, you can say that in your opinion you suspect her (and only her), and then list the reasons for your suspicions: that your physician was the only other person who knew, that she asked unprofessional and impertinent questions about who you know/are related to, and then the fact of the timing--that right after that the same people she asked about suddenly knew the information.
Most likely, your complaint would not be enough for them to take disciplinary action against her, but hopefully they would at least have a serious discussion with her. Warning her that in the future she needs to behave in a professional manner (not ask unrelated nosy questions) and maintain customer confidentiality (not go blabbing about things she knows/guesses from who fills what prescription)!
