That's going to present a problem. Activation lock was implemented by Apple to prevent stolen iPhones and iPads from being used. They also hoped that it would dissuade thieves from stealing them in the first place. As a result, only the owner can remove the device from their Apple ID.
Probably your best bet is to provide proof to Apple of the owner's death and documentation that shows you are entitled to ownership (such as a will). Here's the link to Apple support's contact page; someone there might be able to help you. https://support.apple.com/en-us/HT201232