More likely there were already some issues between her and her husband, the au pair observed it, and decided to seduce the wife away.
I’ve seen it happen with a friend of mine. Troubled marriage, and the “girl friend” of the wife - someone she knew from work - was “there for her”, all the time, whispering in her ear how much of a “bastard” the husband was, how she was being mistreated (she wasn’t), not appreciated, etc...
And then one day, he came home and found them (yeah - “found” them). And that was that.
Sh*t happens, unfortunately.
Poor bastard.
They could have asked him to join in instead.