Where do yo get they should have denied him access? I see that no where in a simple reading:
"Access
A customer with a same-day ticket on American or a oneworld® airline in the first class cabin on a qualifying international or transcontinental flight automatically has access to Flagship® First Dining, located within certain select Flagship® Lounges."
I am on a similar itinerary that I booked specifically to use the FFD at DFW. DFW-LAX-LHR in FF. The EP desk told me i should have access at Dallas.