See Request access to a deceased family member’s Apple Account - Apple Support .
"For security reasons, Apple requires and verifies legal documentation before we can assist with a deceased person’s account. This generally includes a death certificate, and might also require a court order or other documentation."
If her phone is passcode-locked and protected by Find My (Activation Lock), and you do not know her credentials, I do not believe you will be able to get into her phone at a retail Apple Store. The verification of legal documentation probably is not done at individual stores, but by some security group within Apple.
Although Apple does not say this explicitly, my guess is that the request might need to come from the person whom a court appoints as executor or administrator of the estate.