Jack Navaar was one of the early live-in lovers of the movie star Rock Hudson.
The name Jack Navaar is repeated in many sources in this sole context. It is a pseudonyn, a made-up name. There is no surname Navaar. The few people indexed in various sources as Navaar are all mistranscriptions. Dozens of writers, including some of the more famous haven't yet gotten the clue. This name was made up by Davidson to hide the identity of the man still then living. Stop repeating his name as if it's a real name!
Some biographers create a pseudonym for a real character in order to protect the privacy of who they are writing about. It's unlikely Jack Navaar, whoever he is, is still living, so perhaps we'll one day soon find out his real name.