@BUZZ has the best solution, it just takes some execution. I recently was able to make it work like so:
(I'll provide the shortest answer I can first, with additional details about what didn't work if you care.)
Download and install Ext2Volume Manager drivers. These allow your Windows PC to read a mounted Linux EXT2/3 file system.
https://sourceforge.net/projects/ext2fsd/files/Ext2fsd/
Complete online (free) registration for the AccessData FTK Imager. I'm using an older version (3.4.3), they're up to 4.2 now. I assume 4.2 works, but I know 3.4.3 works.
https://accessdata.com/product-download/ftk-imager-version-4.2.0
Make sure you've installed and run the Ext2Volume Manager program. It doesn't have to have the GUI actually running, but you do have to authorize the drivers to run. If you've rebooted since installing it, and didn't choose to autorun on Windows startup, you may have to open the program again. If the rest of this doesn't work, it's probably because the drivers aren't installed/activated.
Open the Access Data FTK Imager program and under File, select Image Mounting.
5.) In the "Mount Image To Drive" window, select your RetroPie.img file. It can be stored locally on your PC, or accessed over the network (thank goodness.) It's actually fairly quick over the network.
Once it scans the .img file - !!!THIS IS IMPORTANT!!! - under the "Mount Method" drop down box choose "Block Device / Read Only".
Press the "Mount" button and the two partitions will be mounted as new drives, both readable. ASSUMING YOU INSTALLED THE Ext2Volume Manager drivers in step one.
Browse and copy the files you need to wherever you want.
Everything below this is just FYSA.
-There is an option to make the mounted files read/write. It does work, as long as you choose a temp location. However I'll be damned if I can figure out how to recombine the two mounted drives back into one bootable .img file once you've modified them. I'm sure there is a way, and I'm going to work on it. I'll update if I figure it out. At least now you have a way to extract copies of files (i.e. video snaps, images, ROMs, etc.)
-The alternative to Access Data FTK Imager is supposed to be "Mount Image Pro" (another forensics kit.) It installs, but consistently crashed for me when I tried to open a networked .img file. Additionally, it requires you to register as well, but you only get a 30 day trial evaluation once you do.