Usage of 'hadn't had' and meaning of this sentence

Question

I read a novel(or maybe it's a short story since it is very short) by Alan Titchmarsh called 'Rosie'. It was going well until I stuck at Chapter 4. It writes

How she had managed to be up and out of the house before him was a mystery. Nick knew she hadn't had that much to drink, but it had affected her.

The question is, is him was a mystery gramatically correct?

And how do we use hadn't had correctly and its meaning in this context?

Answer 1

It appears that you are misparsing this. Him is not the subject of the main clause X was a mystery; that subject is the entire free relative clause How ... him.

^subject[How she had managed to be up and out of the house before him]  ^verb[was]  ^complement[a mystery].

Had[n't] had is an ordinary past perfect construction, indicating that her drinking occurred before the time the author is currently talking about.