You are in a difficult situation,fruitsalad
Such flagrant disregard for what the child witnesses leads to me to think Mother and Boyfriend will not be reasoned with
Any confrontation by you or your husband,will I feel only end in causing distress to you.
Better to let professionals deal with them.I'm sure the local Police will handle the matter sensitively,without bringing your name up.
If the Boyfriend has any sense at all,he will accept all the blame and charges to follow.
Good luck to you both.