I would let the interviewer know that there was an issue with her in the past, that you were inadvertently was involved in. That in contacting her, she may not give a favorable reference due to this.
In our state, employers are limited to what they can answer in regards to a previous employee, and you can report them if they don't follow the rules. But if they are calling her privately in regards to you, and she answers questions, I don't know that you would have much recourse.
The interviewer would also have to tell you what they asked her, and what her response was. Many times, they are not willing to share this information...