Quote 4
"Dead. Or in a crazy house. Or married. I think she's married and quieted down and maybe right in this very city" (1.31).
This is the narrator's answer to Joe's question about Holly's whereabouts, and it's pretty significant that his first instinct is to think that she's dead or locked away somewhere. It seems she's been rather successful in convincing the world that she is meant to spend her life alone.
Quote 5
Only: what other friends of hers did I know? Perhaps she'd been right when she said she had none, not really (15.7).
The narrator thinks this after Holly's arrest when he can't get a hold of anyone to help her. He realizes that her isolation is so complete that only he and Joe are concerned about her well-being. This is one negative consequence of her isolation.
Quote 6
But the address, if it ever existed, never was sent, which made me sad, there was so much I wanted to write her (19.1).
In the end, the narrator just misses his friend a great deal. He wants to share the accomplishments of his life with her, and he feels her absence in a very real way.