Thank you!
The technique is rather simple really, but very clever in my opinion. Also, pretty much any version of PS should be able to do it since it only requires a couple layers, the Clone Stamp Tool and the Quick Select Tool. Here's a brief explanation...
.....Open your file.
Duplicate your Background Layer.
Create a new, blank layer and position it between your Background Copy and your Background.
You should now have three layers; from top to bottom they should be: Background Copy, Blank Layer and Background.
Hide the top layer, the Background Copy. You should now have your bottom two layers visible.
Click to activate/start working on the Blank Layer.
Select the Clone Stamp Tool: 100% Opacity, 100% Flow, Sample All Layers.
Select a clean source to clone from and start cloning out the flash shadow.
BE SURE you wipe your brush OVER the subject a little... No need to nuts here but clone OVER the subject.
Do NOT Panic.
Here's a quick screen shot to show what things will look like at this point. I've worked from the head down to the elbow here. Clearly I could have done a better job with selecting my clone sources in some areas but this should give an idea of what I'm talking about:
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View attachment 307604
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Keep Clone Stamping until you're happy. Subject will NOT look good but that's okay. Just make sure the background of your image looks good and all the flash shadow you want to remove is cleanly cloned out. For the next step I'm going to assume you know the finer points of making and refining a selection, which is what you'll do here. If you don't, that's another conversation. Now, let's get back to work...
.....Hide the New Blank Layer, but keep the Background layer and Background Copy layers visible.
Select the Background Copy to work on.
Using the Quick Select Tool, select the Subject.
Once you have the subject fully selected, SHIFT+Click the "Select and Mask" button on the top toolbar. This will open a Properties tab.
This Properties tab is where you'll feather the selection and generally refine it.
When you're done scroll to the bottom of the Properties tab, find the "Output to" drop-down menu and select, "New Layer With Layer Mask".
Click "OK".
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View attachment 307605
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Selecting your output will take you to your editing screen and you'll see you have a new top-most layer, complete with selection mask. You'll also see the flash shadow but Don't Panic: It's magic time: Hide the Background Copy layer and...
Voila!
You may now delete the Background Copy layer and flatten the image, or continue editing as you see fit.
Hope that helps!