Well, if she's in poor condition, the mane won't grow. A healthy horse grows a healthy mane. An unhealthy horse does not. Once she starts getting groceries, her hair should start coming in. You could also try feeding a supplement. I'd recommend "Source". I've had good luck with it, and it's very inexpensive. You can also pair it with another supplement, like if you're doing a weight/hoof supplement. If her mane is crappy, her hooves are too. The hair she has is poor quality, and the hoof horn she has is probably poor quality too. I'd suggest roaching the mane. Even if she starts growing healthy mane again, the hair she has already isn't going to ever look nice. It's dead and won't benefit from increased nutrients.