In "An American Werewolf in London", why was David able to see and talk with his dead friend?

Jack is cursed to exist in a state of living death for as long as the bloodline of the werewolf that attacked them survives.

You can read the whole plot here. It's not clear how bloodline is interpreted like, but clearly it's not enough for Jack to get the werewolf killed. Probably all the ancestors has to be killed in order to rest in peace.