Could it be possible that the door slammed itself shut between him & Wendy with the differential in heat/air pressure between the apt & hall? Don't fires create a kind of vacuum?(Sincere questions - I know nothing whatsoever about the subject.)
Whatever happened, I suspect the boyfriend will be haunted by this for the rest of his life.
The immense tragedies that have been visited upon this family are so horrific as to be almost unspeakable.
We will find out soon enough. This is such a tragedy for this family.