If it is Marek's, she will always have the disease and many of your other birds too probably and you must consider them all carriers, but she could improve mobility in that leg to a limited or even full extent.... the latter is less likely the longer it goes on. She can certainly survive with her leg as it is with supportive care. I have had one bird learn to lift the leg up out of the way and just hop everywhere .... it took her a few weeks of tripping over her own toes or dragging her foot which caused sores on her knuckles before she figured it out and I tried to help by making a little boot for her but that just seemed to make her more clumsy and trip more. She managed to free range with the flock ang get up and down from the highest roosts.
Your girl will also be prone to future outbreaks. Think of it like cold sores in humans. People who have been infected with the Herpes virus, have it for life but have long spells between outbreaks. Marek's is also a Herpes Virus and acts similarly, it's just that the outbreaks are much more debilitating.
I have had a couple of Marek's birds that were pretty badly incapacitated for months..... one was nest bound for nearly 2 months and eventually got back to free ranging with the flock and laying eggs. That was her second outbreak. Her first outbreak 6 weeks before caused her to be incapacitated for just a few days (she was floundering on her side in classic Marek's splits posture for one day and then miraculously recovered as quickly as she showed symptoms and you could not tell at all.
Some birds are not so lucky and deteriorate and die or are euthanized once they lose interest in food. I offer supportive care as long as they show the spirit to fight it and keep eating.