Am I overreacting? What would you do if you saw a parent slap their kid across the face?

My husband and I were visiting his mother today and she has a 6 year old boy. Well like little boys he started becoming very rowdy and the father of the little boy tells him to shut up and slaps him across the face!! I couldn't help, but to become furious! I seriously want to tell him how much of a sbag he is, but will I be out of line?