Ask Mr. Smarty Plants

Since you didn't specify which ash your parents have on their property, we chose Fraxinus americana (white ash), which is native to Connecticut, to use as an example.  Unfortunately, the members of the Fraxinus genus are somewhat vulnerable to attack by  pests and diseases, prevalent among them being fungi in various forms. The wound to the tree you described will be very attractive to those fungi. 

This information was extracted from a government information paper by Richard C. Schlesinger on Fraxinus americana

"A rust (Puccinia peridermiospora) distorts petioles and small twigs. Cankers caused by Nectria galligena may cause branches to break but are rarely found on main stems. Heartwood rots may be caused by Perenniporia fraxinophilus, Phellinus igniarius, Pleurotus ostreatus, Tyromyces spraguei, and Laetiporus sulphureus. These organisms usually enter through wounds or broken branches, mainly on older trees."

If anything can be done, it probably needs to be done quickly, while woody plants, such as trees, are still dormant in the Winter. You will get better information from horticulture experts who are closer to the scene than we are in Texas. Here is a site from the University of Connecticut College of Agriculture, New Haven County Extension.  The site has contact numbers and e-mail addresses, and hopefully they can direct you to someone better acquainted with the problems for ash trees in the Northeast. If they feel that there is some hope the tree can be saved, they may refer you to a licensed arborist, who will be trained and equipped to deal with tree diseases.

Fraxinus americana

Fraxinus americana

Fraxinus americana

Fraxinus americana