It depends on the point of view of your story.
1st person - For example, if your story is narrated from Person A or Person B, then you can tell the reader all the intimate details as long as THAT PERSON knows them themselves.
3rd person limited - It sounds like, since your narrator doesn't know A and B's intimate details, your story is probably narrated in third person. If it's third person limited, the narrator isn't inside any character's head but they follow one person in particular. You can tell the reader things that the target you're following knows - so even here you might be able to explain intimate details.
3rd person omniscient - The narrator not only is in nobody's head, but they don't follow any one specific character. They know everything, and they can tell the reader whatever they want, I believe (correct me if I'm wrong).
If the narrator is another character that knows A and B, then they should only narrate what they know, right?
Overall, I would say that the narrator may only narrate what she knows to the reader unless it's told through an omniscient point of view.
Hope this helped a bit!