(edit)Let me try this again:

Call SE to see if they can unlink the account from her PSN.

If they can then you just need to add the PS4 code to her account through the PS4.

If they can't just make another PS4 account, and attach the game there. It's a small hassle, but not totally bad. I have to do this because we didn't know it linked to PSN account, and my brother's FFXIV is on my PSN lol.