Lyric discussion by rhimansworld 

Cover art for My Boy Builds Coffins lyrics by Florence + the Machine

As well as apparently being literal, i think it could be metaphorical, like 'her boy' 'makes coffins' i.e puts in bad places od kills or ruins everything for everyone. He's 'made coffins' for her, as in he's killed her spirit or ruined something for her. Just my two pennies worth

That's how I see it, too...