I was regularly punched by my dad aged 13-16. My injuries where either none, a black eye, twisted arm etc.
My mother stood back and watched it happen, infact she would make up lies to make my dad so angry he would hit me.
My dad always reacted in fury. I was a mouthy teen who thought I knew best and I Knew exactly which buttons to press to get the biggest reaction.
Im 28 now and my dad and I are very close.
It took years to repair the damage our relationship suffered.
Looking back dad was working 18 hour days, in an unhappy marriage and coming home to world war 3 every day. He was stressed and had my parents not divorced when I where 16 I do believe he would of ended up dead, or in prison.
My dad cant even speak about this time now, as it brings him to tears. He is a model father and a grand father now.
Op - your daughter needs your support, not only for protection but to also try and mend the relationship. She needs to know its not her fault regardless of what she said and did - I would maybe have the conversation with her that her disrespect pushed the wrong buttons and everyone has these buttons even perfect parents but reiterate that anything she does or said is not excuse for any person to strike her.
I would also support your DH to discover what made him do this? What is the cause, you say he has never done this before and just snapped so something must be on his mind. He owes it to his family to sort this out. He needs to apologise and be the adult and build bridges. Not excusing himself but admitting he needs help and he went to far.