I’d have to try it but I seriously doubt it, the files for the main game aren’t in the Steam folder, the steam client has no idea where they are so wouldn’t be able to delete. I think he’s deleted them in error, maybe he thought he was deleting the cache, by default MSFS files are placed in the AppData folder where all other cache files are kept. I think he has tried to clear his cache and inadvertently deleted his game.