Gamesharing problems on PS4

I’ve been overwhelmed with the unforeseen “hic-up” in my plan as I was trying to GameShare/swap, also I should add that I am ignorant to game swapping because I had a hard time looking up an up-to-date DIY tutorial, therefore I am admittedly afraid to take further action.

So I’d like any game swap masters to please help me out on this subject and hopefully salvage my “gameshare experience”.

Anyway, my cousin came down to my place the other day and I talked him into giving me all his login info to his Playstation network account and to add said Playstation account to my Playstation – in exchange for my word, assuring I wouldn’t mess with his account and only download some games from his massive game/DLC library.

To my surprise I finally got a game downloaded yesterday night and it basically said: “game license is owned by said cousin and I had to use his account because he owned said license.” I’m assuming I cannot just play on his account with no repercussions?

On top of all of that, he was already worried I would somehow ruin his account
(considering the problems I ran into; I can’t say I have the same confidence I had before) and obviously I don’t want to do that.

Therefore is there any way to get around this “hic-up”? What did I do wrong, better yet is gamesharing even possible?

Side note: I’ve had a very hard time finding answers – as many Google searches had conflicting information but on the same token many had the same method that seemed to work – at least from other people as I tried posting on several Reddit threads with no prevail; leaving me uncertain to say the least…
So with that in mind I have not even powered on my Playstation, to assure I don’t ruin any chance I have to “fix” my problem.