A Women spanks her child hard in front of me?

I'm my line of work ( I work for a non-profit organization ) I see many people. I have witness so many single mothers taking out there stress on the child. Ofcourse sometimes a child deserves a spanking, but it just kills me to witness this. Today inside my office this single mother came in with her 4 year old son, she wanted me to return her some money, but im not authorize to do so and I explain, she understands, the little boy keeps on buggin her but in a way any child would, she says ';excuse me'; to me and graves her shoe and starts HITTING HER SON. I complitly FROZE. I mean what can someone do in that situation....I wish there was something, but I really don't know how to handle that...what you think ?