I did a dummy run using BA Avios miles round trip from New York to Seoul in May. I tried it using 1st class and then tried it using business. 1st class available at 300,000 Avios miles per person. Business available at 170,000 Avios per person. Flights would be with BA partner JAL. For years, Diners has run promos once or twice a year to convert Diners points to BA miles at a 50 % premium, so that your father's 300,000 Diners points would convert to 450,000 Avios miles under the promo, if and when it runs again.