What you say is just a scratch is probably actually a dent, which if not replaced, will need some bodywork to straighten, then the damaged area will need to be repainted and then more of the panel recleared and blended in, or if it's single stage paint, at least a large area of the panel repainted and blended in. Then a final wetsand and buff to smooth it all out.
Or, if the bumper is getting replaced, even if it's a used bumper, the whole thing will need to be sanded, painted, wetsanded and buffed, as well as removal and installation.
Even if she wasn't supposed to do a U-Turn, sudden stopping is not illegal, and you rear ended her so I'm affraid legally you were tailgating by not allowing enough distance to stop.
Reading all this back it does sound like I'm being harsh, but I'm just trying to be as honest as I can be. Lying and telling you what you want to hear won't help. From what you've told us $900 does sound a tad high, I'll give you that, but professional body work is expensive. It would have helped a lot to know what year, make, and model of car we're talking about here.