What made the vet say Mareks? What is happening with her leg? What is the antibiotic s/he treated her with and for what diagnosis?
We lost two hens to Mareks, vet verified, but they had significant issues and were not vaccinated for Mareks. I am not a very experienced chicken owner myself but after our experiences with Mareks, I would be asking the vet some questions. We found a non avian vet who took the time and compassion to help us. He spent a half hour on the phone with me after examining our hen to discuss this topic, her condition and options we had. There is no treatment for that disease and long term prognosis is grim if it truly is that - but all other illnesses and diseases should be ruled out first.
There are many other people on this site much more experienced and smarter than me who can help you identify what might be wrong and how to treat it. Just wanted to share our experience.