It appears that your Dad's peach tree has peach leaf curl, a fungal disease caused by the fungus Taphrina deformans.  This organism overwinters in the bark and bud scales of peach trees and then fungal spores infect the leaf buds and leaves in the spring.  This condition does not usually cause death of the tree.  However, significant leaf drop can mean trees are especially susceptible to drought stress and winter injury.  It is suggested that water be applied (an inch a week) during dry periods. Nitrogen fertilizer can be applied up to August 1.  Also, if heavy, thinning of the fruit is recommended.  To try to prevent or reduce this problem in the future, a single fungicide spray in the fall after leaf drop or in the spring before bud swell can be applied.   The following publication lists ingredients to look for in a fungicide.  Also look for a fungicide that is labeled for use on edible plants and be sure to read and follow all directions on any chemical control used.
    

Peach Leaf Curl – Wisconsin Horticulture