He needs to contact Dell Customer Support, give them the Service Tag from the PC, & (assuming he registered it) they might be able to help him. The other alternative might be Microsoft Customer Support, if it's a problem logging into the Windows account.
No one here can help you, though, because a) we're not affiliated with Dell, & b) we have no way to verify your grandfather is who he says he is/is the person that the PC belongs to.