I bought an iphone first owner forget her iphone user and password

See if she can recover the ID/password.

Apple ID - How to find your Apple         An iCloud e-mail address is an Apple ID.

Apple ID - Find

Enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage

Apple ID – Forgot Password

If she can't recover the ID and password to log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from her account, the device is useless. Return it for a refund.

Activation Lock - Turn off Find My iPhone