I was originally going on the assumption that she was in really, really bad shape b/c of the warnings the ER gave her husband about bracing himself. But I now think that the ER staff overexaggerated; especially in light of the fact that when Sherri flagged down that woman, AD, AD said that she didn't look injured. It makes perfect sense now that the hospital wouldn't want to keep her, and that SP and KP didn't feel it necessary, either.
It is possible that due to the darkness and her driving by, that AD didn't get a good look at SP. From what I recall KP mentioned that he was quite shocked when he first saw SP and the extent of her injuries.