If the iPhone is actually signed into her account then you are encountering this feature or a related service:
'Activation Lock for iPhone, iPad, and iPod touch' is a "feature that's designed to prevent anyone else from using your iPhone, iPad, or iPod touch, if it's ever lost or stolen." To learn more click here --> Activation Lock for iPhone, iPad, and iPod touch - Apple Support
In most cases to remove an activation lock absolutely requires the information for the account that applied the lock. Click on this link to learn 'How to remove Activation Lock' --> How to remove Activation Lock - Apple Support
As far as Apple's system is concerned, the owner is the ID that was set on the device. If she set her ID on the device then it will need her information to remove it. The device is otherwise useless to you unless you can get Apple to remove the lock (see below).
Here are additional resources if you need help with lost information:
Help with a lost Apple ID or password:
• "If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> If you forgot your Apple ID email address or phone number - Apple Support
• "If you forgot your Apple ID password" click here --> If you forgot your Apple ID password - Apple Support
Reset device:
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide details as to what is acceptable proof. Prior to this new online service Apple typically required the original receipt* issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).
*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support
However, you also say something about the account not being supported. If the device is really signed into your account and you are getting that message then contact Apple security: Contact Apple for help with Apple ID account security - Apple Support