New Brevoort Formspring

Are there any specific rules or restrictions that come into play when a writer introduces or uses a character based on a real person? After reading the AV Club's article on how Frayda Kafka described it as "very upsetting" seeing Dr. Ashley Kafka, a character clearly based on and inspired by her, killed in the comics and changed into a male Nazi in Amazing Spider-Man 2 I'm curious how this kind of situation works.