Malfoy's offer to move into his house after the Battle had been... unexpected but welcome. Neither he nor Ron had had the guts, or the energy, to go back to the Burrow or Grimmauld at the time. They had both needed to be away from the grief and the fear and the overwhelming feeling of loss but couldn't bear the thought of being apart, not after so long together and, in Ron's rather blunt words, he hadn't trusted Harry out of his sight, in the event he got "some other hair brained idea of self-sacrifice and stupidity." Harry couldn't really blame him. Added in the fact that they all three had noticed and agreed (in a very strange turn of events) that something wasn't quite right with Hermione and wanted to keep a collective eye on her and the easiest way to do that was... together. In the same place. There had been some growing pains, of course, and rather a lot of tension in Green House, particularly with Malfoy on house arrest without a wand and Hermione being... what she had been. But they'd managed. And then she'd been gone and everything had (quite literally) gone up in flames. And now here he was, seven years later, trying to come to terms not only with Hermione's departure and subsequent return but her impending... death.