This is what I think could have happened.
When an owner gets an inquiry it is in HTML format with an orange reply button (which leads to an owner login). If you want to reply through HA you click on that box, login and reply.
If you reply through your email program that button can show up under your response (or as a live link if you switch to plain text). I think that's what she clicked.
I usually switch to plain text and delete all the HA junk, including that reply link.