Activation lock

Activation Lock can be removed however you will need the password to the Apple ID that was displayed on the screen. The seller will need to give you that Apple ID's password so you can enter it and remove the Activation Lock.

If the seller is unable to provide the credentials then I recommend that you return that iPhone and get your money back.

Axel F.

So she will need to recover her apple id password - but it is not guaranteed to be successful.

Either way, you should give the problem back to her. I would ask her to refund you, but say you will buy the phone again if she is able to remove the activation lock.