My friend and I shared ownership of a dog. When I was gone his nurse talked him out of the dog and he wouldn't tell me who he gave him to. I didn't know until a couple months later after he died where my dog went. And now she won't return any of my messages. I want my dog back.
I do understand your wanting your dog back, and I sympathise, but what the dog needs is a stable, predictable environment. If he is now in a good home, the right thing to do for his welfare is leave him where he is.
What sort of messages are you leaving that the nurse won't return?
Come to that - how do you know the nurse has the dog, rather than took the dog to a rescue/rehoming center because your friend couldn't take good care of him and you weren't there?
Recently had a case with our dog rescue. Dog was living with homeless person in park. Homeless person incarcerated, and gave dog to another homeless person. That person gave the dog to rescue and it was placed in a home. Despite the rescue clearing it through Animal Care and Control (one month hold on dog), and getting "ownership", the police showed up at the new owner's home with the homeless person, and the dog had to be surrendered. As the policeman said "In San Francisco the homeless have rights". To my mind, the animals not so much. The dog had been emaciated and scarred from fighting.
So things vary city to city I think. In the end, try everything. I can only say that the "friend" who gave away the pet would not have remained my friend.