You have two options. One is to follow the standard procedure to get access to the account as outlined in this support article --> How to request access to a deceased family member's Apple account - Apple Support
That will also give you access to a restricted amount of data that might be on the device should you want family memories.
The other only gets you access to the device itself. You say you have proof of ownership, but Apple requires very specific proof before they will unlock a device this way.
Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof. Apple may typically require the original receipt* issued by Apple or an Apple Authorized Reseller. If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.
*Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support