It could be on there in several places, including some that will require her account information to remove it.
Ideally get her to use the steps in this support article remove her personal information from a device, even if she don't have it anymore. --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
If you do not want to completely erase the phone (she will have to do it with her account information), you are then faced with the task of going through things one by one. The most common place for this to occur is with apps that were purchased with her account. You have to delete those and download them with your account. If you do not know which ones they are, resort to manual updating one by one and see which ones ask for her information. How to manually update apps on your Apple device - Apple Support
There could be other places too. The nasty one awaiting you might be in Find My. How to remove Activation Lock - Apple Support