In this particular case I was able to copy the notebook file to my own account and open it normally. While investigating the issue I found other users with a similar problem but with notebook files that could not be opened.
There is a known issue where closing a notebook file before it has been properly saved can leave the JSON file with lines such as:
<<<<<<< HEAD
"__version__ = '20230830' \n",
=======
"__version__ = '20230818' \n",
>>>>>>> master
It is the '<<<', '===' and '>>>" lines that are the problem. By opening a terminal and the file with an editor and removing these lines you can fix the file. Hope this helps.
Alternatively, you could recover an earlier version from the '.ipynb_checkpoints" directory but there is no guarantee a checkpoint file will exist.