99.9% of the time, it will grow back just fine. There are some horses that it grows back funky (or doesn't grow back at all) but they are pretty rare. My brother has a horse that consistently kept getting part of his mane pulled out because he was a roping horse and it would get tangled around the horn and pulled out. That has been about 8 or 10 years ago and about a foots worth of mane closest to his withers is still very stringy and scarce. That being said, that is very uncommon so I bet hers will grow back just fine, though it may take a bit of time.