I hope this was a one time mistake and not part of habitual abuse, and that Nick will discuss it with your D before you make any decision. It’s hard to know how best to help him.
In my own experience with an abusive mother, the few times that anyone outside of our family became aware of her behavior she both punished me again for her embarrassment and isolated me from whoever was aware of her abuse.
It may help to talk first not only with Nick but also with his former caretaker. Is your D his only friend who may know about the mother’s behavior? Is there anyone else who to speak with social services reps about the situation? Because you’re dealing with second hand knowledge of something overheard it may not be possible to anyone to act, especially if the mother says your D misunderstood or is mistaken and Nick is too frightened of the consequences to speak up for himself.