Is this a raw file or ? If raw best results will be had in ACR.
Also, make exposure for person not background.
If it's a print (and not a raw file), it is possible to do multiple-exposure scans (5 exposures from very light to very dark) and construct an HDR image from those using File>Automate>Merge to HDR Pro... This produces a 32-bit file, which as you reduce it to 16-bit, you get Photoshop's Tone Mapping dialog. To keep the background intact, since it is a more normal exposure, you might need to do the HDR process twice, once for the foreground, and once for the background, then compose the two together in layers.
Thanks for the comments - this image is a postive film scanned at 9600 dpi 24 bit color - am I to understand that I should scan the film using various exposers?
Why don't you use the <Shadow / Highlight> feature?
> Image >Adjustments > Shadow/Highlights.
Yes, with film pos, this should work beautifully. This technique is something I developed on my own, and its success or failure for you could greatly depend on your scanner software. But it's such a powerful technique, I found it worthwhile to fool around with it until I got great results. I use Silverfast AI (scanning software), which allows various exposures (in Silverfast AI, it's called "Midtone Brightness"). I typically scan 5 times, with exposures set to -100, -50, 0, +50, and +100 naming the scan files by exposure. Then when using Merge to HDR Pro in Photoshop, when manually setting EV, I choose the "EV" option (as opposed to the "Exposure Time" option), with the 0 (normal) exposure set to 9, and each exposure under that -1.5, and each one over that +1.5. -- So -100 EV=6; -50 EV=7.5; 0 EV=9; +50 EV=10.5, and +100 EV=12. This procedure gets GREAT results in constructing a 32-bit HDR image from scans.
An even better method (using the same scans) is to get the plug-in, "HDR Efex Pro 2," which costs $99.95, but has a 15-day free trial period with full functionality. You can see all its amazing capabilities at http://www.niksoftware.com/hdrefexpro/usa/index.php?view=help/videos.shtml
Since getting HDR Efex Pro 2, I don't use Photoshop's Merge to HDR Pro anymore. :+)
With a rescue this severe, you're going to need to do some real trickery. Sure, things like Shadows/highlights are a start, but you're probably going to need some other tricks up your sleeve, such as extreme noise reducition, applied selectively, possibly even copy eye details, etc. from another image. I don't think there's much left of the color in the shadows either, so you may again have to go to another photo for some of the color. Finally you can hide some of the artifacts brought up from the shadows with a bit of trickery, such as a Gaussian Blur that's been faded back some to give the photo a "dreamy" feel.
I don't think it will ever look right if you try to brighten her to "normal" brightness and contrast levels, but you might be able to get enough detail back to at least have the person be recongizable to loved ones.
Here's an example of some of what is possible with this image and a little TLC... Obviously you'd want to use her own eyes from another photo of her.
If it's a tiff or jpg, open it in Camera Raw and play with the sliders. Shoud produce acceptable results.