My father passed away a year ago in December. My mother just found his iPad and we have no idea what the activation passcode is... what can we do to make use of the iPad?

My father passed away 12/8/2019. My mother whom is 85 with limited computer skills just found his iPad and has no idea what the Apple username or password are to activate the iPad.. what can we do to be able to make use of it The iPad was a gift many years ago.. no receipt found.