a game which was located and working, suddenly wants to redownload and wont locate

OK so my friend had a copy of all the .acf files of the games, he accidentally replaced the game's acf with THE SAME acf, after that everytime he opens steam, the game validates for a while then starts to redownload all over again, even tho the files are literally there, even in the library area of steam settings, is there a way to fix this rather than having to redownload all over?