Well, here's how I'd attack it. I know I'm not the most advanced Photoshoper, so there could be a much better way.
Without seeing the original, it's all guessing, especially on an abstract piece. Is mine or Dave's more accurate? Which method is best? Who knows, it's each our 5 minute interpretation of how we think it should look. All we know for sure is the original photograph looks wrong. I definitely agree with Dave that it's worth the trouble of having it re-shot. If they have one dedicated photographer that they use, he should be able to get the lighting better..... But I know that might be hard to say. You can always make a bad image look better, but starting with a good image will get you better results every time. And, I really hope he took a high megapixel image!
A soft light layer can be used to selectively darken an image, and in this case that works well combined with the gradient tool. In a soft light layer, dark is going to darken what is underneath it, 50% gray won't change it, and white is going to lighten what is underneath it.
To start, I want to darken the left of the painting. I set my foreground color to black (LAB 0, 0, 0) and my background color to 50% gray (LAB 50, 0, 0). I created a new layer, made it the current layer, and set it to soft light blend mode. To use the gradient tool, we're going to click and drag, with the initial click at the point we want to start the gradient at our foreground color (black), and releasing at the point we want to end the gradient at our background color (50% gray.) We want the gradient to only change horizontally, so when we're moving the mouse to the release point, we need to hold down the shift keep to ensure the cursor creates a straight left to right line.
This step is definitely subjective, and would require seeing the original to get accurate, but eyeballing it can make it look substantially better. I chose to start the gradient 37 pixels in, and stop it at 406 pixels. (You can open the info panel, click the down arrow and choose panel options, and change mouse coordinates ruler units to pixels to see where you're at.)
If you feel the positioning of the gradient needs to be changed, you can just draw it again.
If you feel there's too much darkening, you can either make the foreground color a very dark gray, or you can leave the gradient black to 50% gray and lessen your opacity percentage. I feel there's too much darkening, and was happy with an opacity of 85%.
You can continue creating additional layers and creating additional gradients. If you want to lighten, change your foreground color to pure white, and draw a gradient in a new layer.
I feel like the top right needs some darkening, so I drew a small 45 degree gradient there, by holding down the shift key and going at the 45 degree angle, although I lowered this layer's opacity to 22%. I feel like the top left needs some darkening, so I drew a small 45 degree gradient there as well, lowering the opacity to 25%. Last, I feel like the bottom left needs some lightening, so I changed my foreground to pure white and drew a small 45 degree gradient there, lowering the opacity to 30%.
If you want to do non-gradient type adjustments, you can of course make a soft light layer and use a 0% hardness brush, painting white, black, or a gray in localized spots. If needed, you'd want to also experiment trying a low opacity brush, like 10%, and using multiple strokes to create a more even blending.
And, this is just a starting point to get the brightness in sync. As I'm sure you know, there will need to be a lot more correcting needing to be done including colors.
