Even if it was a lawyer who contacted you, I think you might have a good argument that she abandoned her property. If you refuse to give it up, she'll have to sue you in small claims court for possession.
Thomas A. Schaeffer, Esq. Law Office of Juarez and Schaeffer PO Box 16216, San Diego, CA 92105 (619) 804-4327 www.jslaw.org This posting is provided for "informational purposes" only and should not be relied upon as "legal advice." Nothing transmitted from this posting constitutes the establishment of an attorney-client relationship. Applicability of the legal principals discussed here may differ substantially in individual situations or in different States.
If you want to keep the dog, wait until you are actually sued for return of the dog. Then counterclaim for all of the expenses (including daily board) for your care of the dog. "Abandonment" is often very specifically defined by statute, but it is possible that an abandoment argument will work. Make sure that you have all your ducks in a row to show ownership (microchip, licensing, veterinary records). If you have any written proof of her giving the dog to you, that could be helpful. You should consult an attorney. These are tricky arguments to make.
This is not to be construed as legal advice. I do not have an attorney client relationship with you.
Send the attorney a letter, with delivery confirmation, and tell the attorney that if your friend wants the dog back she is going to have to sue you and if she sues you you will counter-sue for all your expenses in caring for the dog (food, vet, etc) as well as a daily boarding fee for the amount of time the dog has been with you ($25 - 35 per day). Tell the attorney that you are not to be contacted again except through a lawsuit and that if your friend contacts you otherwise you will go to the police and file harassment charges.
If we do not have a signed fee agreement I am not your attorney and this is not legal advice.