Given what you say, your only possibility for getting the activation locked removed is for the prior owner to have their purchase receipt for the device from an authorized Apple seller with the device’s serial number on it. If they have it, then they may be able to persuade Apple support to remove the activation lock. Have them use support options below to persuade Apple support. Alternatively, if an Apple Store is open in their area, they can go there to deal with the problem. In this case, they can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if desired.
Apple ID Support https://support.apple.com/apple-id
Apple Support Twitterhttps://twitter.com/AppleSupport
Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.