forgotten password and email account no longer exists. how can my wife prove her identity without the correct security answer?

So I received a new iPhone and Instead of setup manually, I did a backup from iTunes. Stupidly I backed up my wife's existing Apple ID account onto my phone!

The password nor security answers are recognised which we know are correct, and to top it off the email address linked to this account no longer exists!

As she previously had 'Find my phone' enabled, this was also passed onto my new phone which prevents me doing a hard reset without Apple ID password 😒

Does anyone know how else we can prove identity to Apple so we can change her email address and reset the password?

Thanks!