Human B complex vitamin tablets are fine, or you can use a poultry vitamin, such as Poultry Cell 1ml daily.
Many conditions can resemble Mareks, and some BYCers have chickens with Mareks who continue to live. Some can die soon after symptoms are seen, some may linger, and some may seem to recover, and later possibly relapse. Some chickens will starve themselves or suffer from infections, but some may eat well. Lameness can effect how much they eat and drink. The important thing is to support her with what she needs, since no one is able to be sure that she has Mareks at all until you can get her tested after death. She could surprise you and make a recovery, at I hope so.