To the best of my knowledge, there is nothing in his contract that excludes HPDE coverage. I am pretty sure they denied his claim because they argued that HPDE driving took place on a race course (without making any claims about him actually racing), but I will ask him. He has studied the contract in detail, so there is nothing that he has missed as far as I know.
The thing is that HPDE events are in a gray zone, and the insurance company will try to leverage that as much as it can. I am not aware of any regular car insurance policy that explicitly states coverage for HPDE events. Please let me know if you know of such a company as I will switch over promptly. A contract cannot possibly cover every single driving scenario, so there will always be gray zones. What about jumping over buses for instance (not as an exhibition but as something you do on your own on your private property)? If someone decides to do something with their car that is unusually risky and is not referenced in one way or another in the contract, what does that mean? I understand your example of driving at 120 mph on a public road also falls into that category and would be covered, but that is somehow different from jumping over buses.