She is signing out of one ID on her device (actually you are doing it for her using the instructions that Phil0124 provided), and signing into a different one on that device. The device is being associated with a different account. That triggers a wait period for some very specific things. That's what that remaining 13 days wait means.
View and remove your devices that are associated with Apple ID purchases - Apple Support - Read "More about associated devices" at the end where it refers to the 90 day wait. This applies to re-downloading past purchases (you can make new purchases), automatic downloads, and iCloud based services such as Family Sharing, Match, and Apple Music.
She can sign in with her own account now. She just has to wait before being able to do some of the things listed in the support article to which I posted a link.
Ideally you and she should be doing the things listed in this article --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support That will remove all of your identity from the device and she will start with a like-new device. I suspect the wait period will also no longer apply.