I am sorry for you loss, this forum is just a user to user forum, no-one here has any account access, on the link below, Microsoft provide the steps to access the account of someone that has passed away, it is basically a legal process that must be entered into to access that account.
Otherwise, if you know the Email address that person used for their Microsoft account, you could try using the steps on this link to reset their Microsoft account password to try using that to access their device.