I mentioned this before in my post #13 but I am going to say it again as @silpat has above but more context is needed and hopefully can be obtained by your daughter talking to Nick prior to any discussions or courses of action being taken.
I will say that I was slapped by my mother as a teen once for some backtalk and I would not consider my mother abusive at all. I was being a brat and she was probably stressed and I’m sure she felt bad about it. More info is needed before making a leap that Nick is being abused based on one incident. Given his dependence on his mother it is worth looking into further, however.