Crazy question, I know. And no, this is not "how 2 get steam gaems for free", I fully expect them to show up with a "Buy Now" button (or something of the sorts) instead of the usual "Play" one.
Simply adding all the files to my "common" folder doesn't do anything. Where, perhaps, does Steam store the data of what games are installed for me?
Steam – How to get Steam games that I haven’t bought to show up on the library
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Best Answer
I'm afraid this can't be done.
It's seems that Family Sharing authentication isn't given to a Steam-account, but to a "machine". And a different Steam installation (as well as a resintall) counts as a "new" machine, as far as Steam is concerned.
As such, the reinstall has wiped out the Family Sharing authentication that your friend has given earlier. Steam is actively ignoring the files you copied, because it no longer thinks you have an authentication to play the games.
I'm afraid that the only solution is to have your friend somehow re-authenticate your "new machine" through Family Sharing. Yes, this means somehow getting him in front of your computer again.
Source: https://steamcommunity.com/groups/familysharing/discussions/1/540738051790971117/