|Distorted Mulberry Shoot|
So the most likely cause of this damage to his tree was a neighbors spraying accident. 'Phenoxy' type herbicides (those containing 2-4 D and similar compounds) are notorious for causing damage to trees from spray drift caused by applicator error or applying the spray in windy, hot conditions. All it would take would be a neighbor spritzing weeds in his lawn with a hose end, pump, or hand pump sprayer in a light breeze; and to top it off Mulberries are notoriously sensitive to these types of herbicides.
The treatment? If the accidental dose is not to great the tree should grow out of the damage either before the summer or next spring when the tree re-leafs after its winter dormant period.