How serious/dangerous are sun blisters?

Sun damage to the extent that the skin blisters is dangerous. As your dad has been exposed to so much sun, he is at greater risk of skin cancer. He should get this blister checked by a doctor. He should also make sure to protect the tops of his ears as this is a common site for skin cancer. Tell your dad that you need peace of mind and you won't get it until he sees a doctor.