If you have a service account, you can register PC and PS4 game keys to it so when you log in using the log-in for that account is acknowledged to have access to whatever is tied to that specific Service account. Only one PSN ID can be tied to a specific service account,

In order for your GF to play they will have to either start a trial account, using the trial version of the game as previously mentioned. Or create a brand new Service account on the Mogstation or via the start-up on a new game, and add a new Game key(from a new purchased game) to that service account on the Mogstation. It is against TOS to share any one account.