I'm not sure to understand. If you didn't bought the game on steam or itch and your friend gave you the file, it looks you got a pirated version of the game.
Viewing post in Didnapper 2 - Yuti's mod comments
That's good to hear :)
I'll be happy to help you if you still have troubles installing the mod on your copy.
I would suggest buying it on itch.io because I noticed sometimes Steam may mess up files between two updates which require to reinstall the whole game to make mods working (not hard, but anoying).