This (silent) video illustrates Sunny-Dave's suggested method:
https://tinyurl.com/ybng62lc
However looking at it suggests to me that your OH is likely to lose all his bookmarks and personal settings unless he backs them up FIRST. So, to be on the safe side, he needs to do the following BEFORE uninstalling and reinstalling:
Press the Windows key and R together.
Type this text (or copy & paste it from here) into the box:
%APPDATA%\Mozilla\Firefox\Profiles\
Click OK
Right-click on the folder with 'default' in its name and select 'Copy'
Go to a convenient location (e.g. the desktop), right-click and select 'Paste'.
Once he's done that he can then follow the instructions given by Sunny-Dave (and shown in the video). I always regard it as good practice to RESTART a computer between uninstalling a program and then reinstalling it; it flushes the registry to get rid of any last traces of the program.
Once he's managed to reinstall Firefox he should take a look to see if his bookmarks, etc are still there. If they are he can ignore the back-up file he placed on his desktop and delete it.
If his bookmarks are missing though he should right-click on the back-up on his desktop and select 'Copy'. Then he should repeat the process above to find a file with 'default' in its name. He should DELETE that file and then right-click where it was and select 'Paste' to put the old profile folder there. (He should then have his bookmarks back and he can delete the backup file on his desktop).