iCloud activate for the iPhone 7plus have been using for two years

Hi how are you 

I have Iphone7+ I have been using for two and half years I bought it second hand now just yesterday I thought of selling it to a friend because I bought a new iPhone hence I did factory reset not knowing the first owner did not removed it from there iD so now for me to start over sadly it requested saying this phone is linked to Apple ID please enter the password, I called the first owner he doesn’t even know where to start because he knew not even a single thing on iPhone because he just used it once and go back to androids all he can remember is a Apple ID and the password but it’s saying this Apple ID has been disabled due to security reasons now the information needed to reset the owner doesn’t remember anything we tried using his NRC card but it’s like they put different birth date not exactly from nrc when creating the account 

Please help me how can you do on that one?

Thank you