Should I confront this guy for hitting his dog?

I work for a commercial cleaning company. We clean office buildings. There is this guy who keeps his dog in his office with him. I don’t know his name but we say hi to eacthother. Anyways, when I went into his office to empty his trash, his dog starred barking. He was like no! Then slapped her on the side. Not enough to hurt her but it made a slap sound. I didnt say anything and I feel kinda bad. Should I say something to him now?