Dog stolen-what can i do?

We adopted a dog who was clearly loved by his previous owner. She had twins and had to move into a condo. The dog requires daily exercise and a fenced yard because it's a very energetic breed. She had him for 4 years. When we adopted him she asked if she'd be allowed to visit him and we said yes. WeWe had him for 2 months and she contacted us saying how much she missed him and that she wanted him back. We have grown close to him and we didn't want to give him back. She has been calling, emailing and texting us for months now saying she wants to visit and see him... Im afraid that if we let her see and chair him that she will take him or will steal him from. What can I do? What can I do if he is stolen?