Well, it's an unclear structure. Putting a comma before and after someone's name is correct if including the name would not change the meaning of the sentence, as in this case. In the case of OP's sentence, it could have implied that Polidori was Mary Shelley's husband as his name, while important to the meaning of the sentence, was included in a list of other people, including Mary Shelley, Mary Shelley's husband, and Lord Byron.