Log out of your account in System Settings ➜ Apple ID, then when prompted to sign back in click on the Create new Apple ID option. click here ➜ Create an Apple ID on Mac - Apple Support
With that done, open Messages, click on the Messages menu, then on Settings then on @, and make sure the new Apple ID is logged in there.
Repeat in FaceTime ➜ Settings ➜ General
And in the Apple Store App ➜ Store Menu ➜ Sign Out, and sign in with the new Apple ID.
Do note, any apps you may have downloaded from the App Store will forever be tied to your account, and require your account to update. OS updates may also require your account and password.
Really, the best option would have been to create the Apple ID during setup for her, instead of using your own.
You can erase the Mac entirely, and set it up again, and create the Apple ID then, so nothing is tied to your own account.
click here ➜ What to do before you sell, give away, or trade in your Mac - Apple Support