lost apple id

You need a copy of the original purchase receipt for Apple to unlock the phone. You also a copy of the death certificate and a copy of the will or a document that shows you inherited it.

How to request access to a deceased family member's Apple accounts

Sorry, but you'll have to contact Apple Support about that and ask them for help.

They will tell you what information they will need to disable the Activation Lock.

Apple ID - Official Apple Support