Old Phone Activation Problem.

So I gave my old 6Plus to my Mom, who then returned it without signing out everywhere. Now I can't activate the phone as it says it is linked to her ID. She doesn't use apple anymore and can't remember the ID or the password. We attempted a "Forgotten Password" but the ID she used apparently now does not exist on apple. I removed the phone from my account when I gave it to her so I have no "old" backups. What to do?