Activation Lock

Good day, Apple community. My mom bought an iPhone 6 plus from someone. When we opened the device, we were stunned to find an activation lock from someone's email. We tried contacting the previous owner, asking for the refund, but he was not answering my mom's call. We don't know the ID nor the password, and tried to handle it to the shop, but they also didn't know what to do. I tried everything from jailbreaks and hacks, but it doesn't seem to work. Please help me. Thank you.