Hi Rose,
Be glad, she asked you at least. Who knows how many people just give it away without your knowledge. The special discount, you offered, is a smart solution to keep everybody happy.
Do you use a list, people can subscribe to, for updates and additional info and such? If so, compare the recipients with the people who purchased your book. Then you might get an idea, how many people just copied it.
If I am not mistaken, there is a way to lock illegal book files. But I am not familiar with this technique, though. Maybe someone else can help you out.