Hi,
You're probably going to hate me, but this "rescue" probably won't end well. No, if she is a broiler, she can not be free fed any feed of any sort. In fact, I would only feed her very little, and make her forage for food. Somebody else with more experience can tell you specifics as I've only seen a broiler live much past your bird's age once and that bird free ranged and foraged with no supplemental food for months until it disappeared because it somehow got forgotten during loading for the processor. I'm not saying this is ideal (PROTECTION?!!! SHELTER?!!!), but that was how it lived until something got it. Basically, broiler chickens are a multiple generation hybrid cross that is genetically programmed through careful breeding to get as fat as possible as quickly as possible and aren't meant to live very long at all. They basically self destruct. So if you want to keep her alive, it's all about healthy CONTROLLED diet, exercise, and luck. Honestly, if I were you I would either give her back, or save one more so she'll have a buddy in her new, strict dieting life. And if she makes it to laying, you win. Just be aware that if she survives to adulthood it's very likely that she's eventually going to have a broiler problem come back and get her sooner than expected.
There's a thread on here about someone who raises giant Cornish crosses like your bird, basically by working hard to keep them alive for longer than normal until they're huge. I'd look for that thread and glean what you can from it. Your goal is to combat her genetics to be a blob and bring out the chicken in her, because that will very much help keep her alive. I hope the person who wrote that thread/article sees this and gives better instructions.
But in any case, here's a bump.
Good luck. I'd love to know how this challenge winds up.
-R
