When is there a Duty to Help with Unloading and Loading?

Question Chayim sees a man attempting to unload a huge wardrobe from a truck, single-handed. As expected, the wardrobe falls! The man asks Chayim to help him to carry this heavy item into the nearby apartment. Is Chayim obligated to do so? Answer "When you see the donkey, etc buckling under its load, etc. you…