I think they take the rest of it off.
Just out of sick curiosity: What happens to your head in such a case, if they can't reattach the scalp? Do you end up with what is essentially the world's worst facelift? Do they just put a metal plate in there and coax the skin to grow over it?
I would imagine that extensive reconstructive surgery would be needed. It really depends if she was scalped down to the bone or a thin layer of muscle was left. If she was scalped down to the bone, she may have severe problems the rest of her life, which may end up being quite short.
A plastic surgeon told me the full scalp will grow back in less than a year ("rolling in" from the sides), as long as there is no major infection or other complication. Of course, there will be no hair follicles in the new tissue. I have seen a case similar to that myself.
I think they take the rest of it off.
They'll probably take skin from her thigh or buttocks and reattach it to her scalp.