I don't know how medical confidentiality laws apply to porn casting agents or why it would be different than if a person openly shared confidential information with any other person not directly involved in health care.

While medical records are confidential, if a person openly shares his/her medical history in a non-confidential manner, then s/he has voluntarily given up that part of confidentiality.

So, if she shared the results of her confidential test with her agent and he is not a lawyer, I would tend to believe that conversation/exchange is not covered by confidentiality rights. Although it's kind of slimy for an agent, ex or otherwise, to do it.