Can I keep playing games offline from my Cousin Library after he remove his account from my PS4?

Hi Y'all

So my friend is going to Vacation for 13 days and he Told me that I can Keep his Account as Primary in my PS4 for the time being and Play Batman(Using My account for Trophy Collection)

So What If I download a Game and Set his Account as Primary and Go Completly Offline Until I Platinum, does this Scheme work?

Like I am Very Slow Player and I dont wanna rush through Game.

So Even If he Log In after 13 days, Will I lose my Primary Access and Will my Game Lockout?