I am sorry your friend's niece has been diagnosed, but it is good you are helping her. Dr. Chandra Swami in Hermitage, PA comes very highly recommended. I believe he treats children and adults.
If not, Dr. Charles Ray Jones in New Haven, CT is the best Lyme pediatrician. He sees patients from all over and has helped many children. He is worth making the trip. Patients from out of town who see him can stay at the nearby Ronald McDonald House for $15.00 per family per night.
Make sure your friend learns as much as possible. Tell the mother about the co-infections, herx reaction, and the need for proper aggressive treatment. I recommend that the mother read "Everything You Need To Know About Lyme Disease (2nd edition)" by Karen Vanderhoof-Forschner and Dr. Burrascano's treatment guidelines. He is one of the top Lyme doctors in the country, and many doctors follow his protocols.
I hope this child can get the proper treatment as soon as possible. You may want to suggest to her mom to join the site here