Finally, my friend caught the mule and removed the board from the halter rope. He petted the mule and calmed it down. But, it took about two years before this mule had any confidence in this person. When working around mules, be patient. Take your time, don't act aggressively and constantly reassure the mule.
Mules are sociable, gregarious and less quarrelsome than horses.
They bond readily with people if treated kindly and respectfully. Another friend's mule would bray as soon as he came out of the house. If he gave attention first to another animal, the mule got upset. Mules can't be absolutely dominated by people due to their strong will. Some individuals respect this characteristic, while others don't.