Scott, the fact that it happened before you were POA is complete BS. That has nothing to do with anything. I would send a fax (always do this in writing) and let her know that from here forward all communications are to go through you. If there is an issue then please have her manager contact you.
Technically its a TIGTA violation and a violation of the taxpayer bill of rights. It can potentially cost the RO their job, so I would not threaten that yet, but if it happens again so it is now intentional then I would send another letter explaining that to the RO and their manager. The last resort is to file a formal compliant with TIGTA, which is effectively the equivalent of nuclear war, so lets hope that doesn’t become necessary.