Trying to completely reset an old iPhone 6

You will need her help to be able to use the phone. She will need to figure out her account credentials

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

https://iforgot.apple.com/appleid#!&section=appleid

If you forgot your Apple ID password - Apple Support

How to remove Activation Lock - Apple Support

This might be a problem if it is currently locked to her apple ID, and she can't remember the details for this.

Is it showing activation lock, or is it showing disabled?

If disabled, you need to go here

If you've forgotten the passcode on your phone

If it is showing activation lock it is because it was registered to her Apple ID - If you know the apple id logon, then see:

Turn off activation lock

If you don't know her apple ID Details see

If you forgot your apple ID