He must not have bothered to read the entry blanks he signed. Since I’m sure he’s still reading, this is the standard language from the USDF entry form (emphasis mine):
US Equestrian Federation, Inc. Entry Agreement I have read the United States Equestrian Federation, Inc. (the “Federation”) Entry Agreement (GR906.4) as printed in the Prize List for [insert name here] (“Competition”) and agree to all of its provisions. I understand and agree that by entering this Competition, I am subject to Federation Rules, the Prize List, and local rules of the competition. I agree to waive the right to the use of my photos at the competition, and agree that any actions against the Federation must be brought in New York State.