How do I unlock a locked Apple Watch using the owner's email?

Hi, i purchased an apple watch at the facebook market place all the while it was working fine but after resetting i found out it was locked to the owner. The seller told me that it was from her sister that has upgraded to a newer version of the iwatch. Can you tell me please if this email exist in your system, mail@gmail.com. Because he had given me the password and the email for me to erase from her icloud. But the password to it was incorrect. Please help.

[Edited by Moderator]