Jack has used Google Takeout to download all his photos. What are the JSON files? Leo says that JSON is a text file that Google uses to provide a list of all photos. But he should also have all his JPEGs. There may also be metadata. Google has a note to explain it here.
One thing to try and rename the image.json to image.jpg and see if that returns to an image. With thousands of photos, he will want a mass file renaming tool to fix that problem. FileExplorer can do it. Just select all photos to rename first. Here's also a good article about how to migrate away from Google Photos -https://legault.me/post/correctly-migrate-away-from-google-photos-to-icloud